mmc: sdhci: Fix possible Synchronous Abort using PIO mode

mmc: sdhci: Fix possible Synchronous Abort using PIO mode

When MMC_SDHCI_SDMA=y or MMC_SDHCI_ADMA=y and PIO mode is used
dma_unmap_single() is called on an unmapped address, 0x0. This may
result in a Synchronous Abort:

  ## Checking hash(es) for Image atf-1 ... sha256+ OK
  CMD_SEND:16
                  ARG                      0x00000200
                  MMC_RSP_R1,5,6,7         0x00000900
  CMD_SEND:18
                  ARG                      0x00004005
  "Synchronous Abort" handler, esr 0x96000147
  elr: 00000000400015bc lr : 0000000040012b4c
  x 0: 0000000000008000 x 1: 0000000000092600
  x 2: 0000000000000040 x 3: 000000000000003f
  x 4: 0000000000000030 x 5: 0000000000000001
  x 6: 0000000000000001 x 7: 0000000000000000
  x 8: 000000000000000a x 9: 0000000000000090
  x10: 0000000043dffc68 x11: 0000000043c00440
  x12: 0000000043c00440 x13: ffffffffbfe00000
  x14: 000000000000031c x15: 0000000240000000
  x16: 000000004001145c x17: 0000000000000032
  x18: 0000000043dffef0 x19: 0000000043c00000
  x20: 0000000043dffbc8 x21: 0000000000000000
  x22: 00000000000f3d95 x23: 0000000000000002
  x24: 0000000000000493 x25: 0000000000092600
  x26: 0000000000000001 x27: 0000000000000001
  x28: 0000000000000008 x29: 0000000043dffab0

  Code: d2800082 9ac32042 d1000443 8a230000 (d5087620)
  Resetting CPU ...

  resetting ...

Fix this by only dma_unmap_single() when DMA mode is used and
sdhci_prepare_dma() has been called to map host->start_addr.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Peng Fan <peng.fan@nxp.com>