x86, msr: fix msr-reg.S compilation with gas 2.16.1

msr-reg.S used the :req option on a macro argument, which wasn't
supported by gas 2.16.1 (but apparently by some earlier versions of
gas, just to be confusing.)  It isn't necessary, so just remove it.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Borislav Petkov <petkovbb@googlemail.com>