x86/msr: Make locally used functions static

The functions msr_read() and msr_write() are not used outside of msr.c,
make them static.

 [ bp: Massage commit message. ]

Signed-off-by: Zhao Xuehui <zhaoxuehui1@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210408095218.152264-1-zhaoxuehui1@huawei.com