x86, MCE, AMD: Correct thresholding error logging

mce_setup() does not gather the content of IA32_MCG_STATUS, so it
should be read explicitly. Moreover, we need to clear IA32_MCx_STATUS
to avoid that mce_log() logs the processed threshold event again
at next time.

But we do the logging ourselves and machine_check_poll() is completely
useless there. So kill it.

Signed-off-by: Chen Yucong <slaoub@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>