pata_mpc52xx: reduce code size by simple change of constant data types

I've prepared a totally simple patch that, if I did it and measured it
correctly, reduces the text size as of the ppc-6xx-size command of
pata-mpc52xx by more than 10%, by reducing the rodata size from 0x4a4
to 0x17e bytes. This is simply done by changing the data types of the
ATA timing constants.

If you are interested at all, and it's worth the trouble, here the
details:

ppc-6xx-size:
     text data bss  dec  hex filename
old: 6532 1068   0 7600 1db0 pata-mpc52xx.o
new: 5718 1068   0 6786 1a82 pata-mpc52xx.o

The (assembler) code itself doesn't really change very much. I double
checked the final results inside mpc52xx-ata-apply-timings() and they
match. The driver is still working fine of course.

Signed-off-by: Roman Fietze <roman.fietze@telemotive.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>