i386: Use functions from library in msr driver

Use safe MSR functions provided by arch/*/lib/msr-on-cpu.c in
arch/i386/kernel/msr.c.

Signed-off-by: Nicolas Boichat <nicolas@boichat.ch>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>