ATA: Squash warnings in mxc_ata.

mxc_ata.c: In function ‘set_ata_bus_timing’:
mxc_ata.c:118: warning: dereferencing type-punned pointer will break
strict-aliasing rules
mxc_ata.c:125: warning: dereferencing type-punned pointer will break
strict-aliasing rules
mxc_ata.c:129: warning: dereferencing type-punned pointer will break
strict-aliasing rules

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>