ata: libata-core: fix parameter type in ata_xfer_mode2shift()

The data transfer mode that corresponds to the 'xfer_mode' parameter for
ata_xfer_mode2shift() is a 8-bit *unsigned* value.  Using *unsigned long*
to declare the parameter leads to a problematic implicit *int* to *unsigned
long* cast and was most probably a result of a copy/paste mistake -- use
the 'u8' type instead, as in ata_xfer_mode2mask()...

Found by Linux Verification Center (linuxtesting.org) with the SVACE static
analysis tool.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>