mpc8xx/fec.c: Fix GCC 4.6 build warnings

mpc8xx/fec.c: Fix GCC 4.6 build warnings

Fix:
fec.c: In function 'fec_pin_init':
fec.c:381:18: warning: variable 'fecp' set but not used
[-Wunused-but-set-variable]
fec.c: In function 'fec8xx_miiphy_write':
fec.c:1013:8: warning: variable 'rdreg' set but not used
[-Wunused-but-set-variable]

Note: The code was slightly rearranged, but no functional changes
attempted, i. e. no conversion to use I/O accessors.

Signed-off-by: Wolfgang Denk <wd@denx.de>