x86: rename all fields of mpc_oemtable oem_X to X

Impact: cleanup, solve 80 columns wrap problems

It would be cleaner to rename all the mpc->oem_X fields to
mpc->X - that alone would give 4 characters per usage site.
(we already know that it's an 'oem' entity -
no need to duplicate that in the field too)

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>