i386: msr.h: be paranoid about types and parentheses

When implementing things as macros, make sure we use typecasts and
parentheses where needed.  The macros as defined were vulnerable to
surreptitious promotion causing problems.

Avoid macros where practical; e.g. wrmsr() can be an inline instead.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>