ath6kl: Fix a debugfs crash for USB devices

Credit distribution stats is currently implemented
only for SDIO. This fixes a crash in debugfs for
USB interface.

BUG: unable to handle kernel NULL pointer dereference at   (null)
IP: [<f91c2048>] read_file_credit_dist_stats+0x38/0x330 [ath6kl_core]
*pde = b62bd067
Oops: 0000 [#1] SMP

EIP: 0060:[<f91c2048>] EFLAGS: 00210246 CPU: 0
EIP is at read_file_credit_dist_stats+0x38/0x330 [ath6kl_core]
EAX: 00000000 EBX: e6f7a9c0 ECX: e7b148b8 EDX: 00000000
ESI: 000000c8 EDI: e7b14000 EBP: e6e09f64 ESP: e6e09f30
DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
Process cat (pid: 4058, ti=e6e08000 task=e50cf230 task.ti=e6e08000)
Stack:
00008000 00000000 e6e09f64 c1132d3c 00004e71 e50cf230 00008000 089e4000
e7b148b8 00000000 e6f7a9c0 00008000 089e4000 e6e09f8c c11331fc e6e09f98
00000001 e6e09f7c f91c2010 e6e09fac e6f7a9c0 089e4877 089e4000 e6e09fac

	Call Trace:
	[<c1132d3c>] ? rw_verify_area+0x6c/0x120
	[<c11331fc>] vfs_read+0x8c/0x160
	[<f91c2010>] ? read_file_war_stats+0x130/0x130 [ath6kl_core]
	[<c113330d>] sys_read+0x3d/0x70
	[<c15755b4>] syscall_call+0x7/0xb
	[<c1570000>] ? fill_powernow_table_pstate+0x127/0x127

Cc: Ryan Hsu <ryanhsu@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>