[POWERPC] 8xx: Remove unneeded and misspelled prototype m8xx_calibrate_decr

m8xx_calibrate_decr seems to be a misspelled prototype for
mpc8xx_calibrate_decr. As it's not needed anyways, just remove it.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>