mtip32xx: fix checks for dma mapping errors

exec_drive_taskfile() checks for dma mapping errors by comparison
returned address with zero, while pci_dma_mapping_error() should be used.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Jens Axboe <axboe@fb.com>