ath6kl: Use mutex to protect dma buffer in sync read write

Firmware crashes while starting Soft AP in 32 bit x86 platform.
The reason is that the single dma buffer (ar_sdio->dma_buffer)
is used in ath6kl_sdio_read_write_sync() for unaligned buffer
handling and this function is called in the multiple context
at the same time. So, finally hits dma buffer corruption and
firmware crash.

Mutex is used to protect dma buffer to avoid data corruption.
Spin lock can not used to fix this issue since mmc stack
read/write calls may for sleep.

Observed this issue with recently commited patch
"ath6kl: Claim sdio function only at appropriate places"
861dd058

kvalo: change name of mutex to more descriptive and add a comment
about what it protects

Signed-off-by: Raja Mani <rmani@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>