mpc8260/speed.c: Fix GCC 4.6 build warnings

Fix:
speed.c: In function 'get_clocks':
speed.c:113:30: warning: variable 'cpmdf' set but not used
[-Wunused-but-set-variable]
speed.c:113:23: warning: variable 'busdf' set but not used
[-Wunused-but-set-variable]

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