x86, MCE, AMD: Make error_count read only

Until now, writing to error count caused the code to reset the
thresholding bank to the current thresholding limit and start counting
errors from the beginning.

This is misleading and unclear, and can be accomplished by writing the
old thresholding limit into ->threshold_limit.

Make error_count read-only with the functionality to show the current
error count.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>