mtd: nand: sh_flctl: use dma_mapping_error to check map errors

The return value of dma_map_single() should be checked by
dma_mapping_error(). However, in function flctl_dma_fifo0_transfer(), its
return value is checked against NULL, which could result in failures.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>