mpc8xx/i2c.c: CodungStyle cleanup

Make (mostly) checkpatch-clean

We don't acctually change the code (like convert to use I/O
accessors), so there will be some remaining "Use of volatile"
warnings from checkpatch.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>