x86/CPU/AMD: Simplify the spectral chicken fix

x86/CPU/AMD: Simplify the spectral chicken fix

msr_set_bit() takes a bit number to set but MSR_ZEN2_SPECTRAL_CHICKEN_BIT
is a bit mask. The usual pattern that code uses is a _BIT-named type
macro instead of a mask.

So convert it to a bit number to reflect that.

Also, msr_set_bit() already does the reading and checking whether the
bit needs to be set so use that instead of a local variable.

Fixup tabbing while at it.

No functional changes.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://patch.msgid.link/20251230110731.28108-1-bp@kernel.org