mmc: cavium: Add the missing dma unmap when the dma has finished.

mmc: cavium: Add the missing dma unmap when the dma has finished.

This fixes the below calltrace when the CONFIG_DMA_API_DEBUG is enabled.
  DMA-API: thunderx_mmc 0000:01:01.4: cpu touching an active dma mapped cacheline [cln=0x000000002fdf9800]
  WARNING: CPU: 21 PID: 1 at kernel/dma/debug.c:596 debug_dma_assert_idle+0x1f8/0x270
  Modules linked in:
  CPU: 21 PID: 1 Comm: init Not tainted 5.3.0-rc1-next-20190725-yocto-standard+ #64
  Hardware name: Marvell OcteonTX CN96XX board (DT)
  pstate: 80400009 (Nzcv daif +PAN -UAO)
  pc : debug_dma_assert_idle+0x1f8/0x270
  lr : debug_dma_assert_idle+0x1f8/0x270
  sp : ffff0000113cfc10
  x29: ffff0000113cfc10 x28: 0000ffff8c880000
  x27: ffff800bc72a0000 x26: ffff000010ff8000
  x25: ffff000010ff8940 x24: ffff000010ff8968
  x23: 0000000000000000 x22: ffff000010e83700
  x21: ffff000010ea2000 x20: ffff000010e835c8
  x19: ffff800bc2c73300 x18: ffffffffffffffff
  x17: 0000000000000000 x16: 0000000000000000
  x15: ffff000010e835c8 x14: 6d20616d64206576
  x13: 69746361206e6120 x12: 676e696863756f74
  x11: 20757063203a342e x10: 31303a31303a3030
  x9 : 303020636d6d5f78 x8 : 3230303030303030
  x7 : 00000000000002fd x6 : ffff000010fd57d0
  x5 : 0000000000000000 x4 : ffff0000106c5210
  x3 : 00000000ffffffff x2 : 0000800bee9c0000
  x1 : 57d5843f4aa62800 x0 : 0000000000000000
  Call trace:
   debug_dma_assert_idle+0x1f8/0x270
   wp_page_copy+0xb0/0x688
   do_wp_page+0xa8/0x5b8
   __handle_mm_fault+0x600/0xd00
   handle_mm_fault+0x118/0x1e8
   do_page_fault+0x200/0x500
   do_mem_abort+0x50/0xb0
   el0_da+0x20/0x24
  ---[ end trace a005534bd23e109f ]---
  DMA-API: Mapped at:
   debug_dma_map_sg+0x94/0x350
   cvm_mmc_request+0x3c4/0x988
   __mmc_start_request+0x9c/0x1f8
   mmc_start_request+0x7c/0xb0
   mmc_blk_mq_issue_rq+0x5c4/0x7b8

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Fixes: ba3869ff32e4 ("mmc: cavium: Add core MMC driver for Cavium SOCs")
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>