MLK-11848-1 spi: spi: Revert "spi: check tx_buf and rx_buf in spi_unmap_msg"

This commit 06634f8a25f4c5e4ff0c0ef5368a48eb394db63b cherry-picked wrong,
and that cause SPI DMA mode never works. Actually, no need this patch since
this patch has already been upsteaminged as below:

commit f8bb820d
Author: Robin Gong <b38343@freescale.com>
Date:   Thu Apr 16 10:54:18 2015 +0800

    spi: check tx_buf and rx_buf in spi_unmap_msg

Signed-off-by: Robin Gong <b38343@freescale.com>