hwrng: caam - fix rng_unmap_ctx's DMA_UNMAP size problem

Fix rng_unmap_ctx's DMA_UNMAP size problem for caam_rng, else system would
report the below calltrace during cleanup caam_rng.
Since rng_create_sh_desc() creates a fixed descriptor of exactly 4
command-lengths now, also update DESC_RNG_LEN to (4 * CAAM_CMD_SZ).

caam_jr ffe301000.jr: DMA-API: device driver frees DMA memory with different size [device address=0x000000007f080010] [map size=16 bytes] [unmap size=40 bytes]
------------[ cut here ]------------
WARNING: at lib/dma-debug.c:887
Modules linked in:
task: c0000000f7cdaa80 ti: c0000000e5340000 task.ti: c0000000e5340000
NIP: c0000000004f5bc8 LR: c0000000004f5bc4 CTR: c0000000005f69b0
REGS: c0000000e53433c0 TRAP: 0700   Not tainted
MSR: 0000000080029000 <CE,EE,ME>  CR: 24088482  XER: 00000000
SOFTE: 0

GPR00: c0000000004f5bc4 c0000000e5343640 c0000000012af360 000000000000009f
GPR04: 0000000000000000 00000000000000a0 c000000000d02070 c000000015980660
GPR08: c000000000cff360 0000000000000000 0000000000000000 c0000000012da018
GPR12: 00000000000001e3 c000000001fff780 00000000100f0000 0000000000000001
GPR16: 0000000000000002 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000000 0000000000000000 ffffffffffffffff 0000000000000001
GPR24: 0000000000000001 0000000000000001 0000000000000000 0000000000000001
GPR28: c000000001556b90 c000000001565b80 c0000000e5343750 c0000000f9427480
NIP [c0000000004f5bc8] .check_unmap+0x538/0x9c0
LR [c0000000004f5bc4] .check_unmap+0x534/0x9c0
Call Trace:
[c0000000e5343640] [c0000000004f5bc4] .check_unmap+0x534/0x9c0 (unreliable)
[c0000000e53436e0] [c0000000004f60d4] .debug_dma_unmap_page+0x84/0xb0
[c0000000e5343810] [c00000000082f9d4] .caam_cleanup+0x1d4/0x240
[c0000000e53438a0] [c00000000056cc88] .hwrng_unregister+0xd8/0x1c0
Instruction dump:
7c641b78 41de0410 e8a90050 2fa50000 419e0484 e8de0028 e8ff0030 3c62ff90
e91e0030 38638388 48546ed9 60000000 <0fe00000> 3c62ff8f 38637fc8 48546ec5
---[ end trace e43fd1734d6600df ]---

Signed-off-by: Yanjiang Jin <yanjiang.jin@windriver.com>
Acked-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>