x86/mce/intel: Use MCG_BANKCNT_MASK instead of 0xff

Use the predefined MCG_BANKCNT_MASK macro instead of the hardcoded
0xff to mask the bank number bits.

No functional changes intended.

Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/r/20241025024602.24318-3-qiuxu.zhuo@intel.com