x86: correctly report NR_BANKS in mce_64.c

attached is a no-brainer that makes kernel correctly report
NR_BANKS for MCE. We are right now limited to NR_BANKS==6, but the
error message will use the available number of banks instead of the
defined maximum.

For a Nehalem based system it will print:

"MCE: warning: using only 9 banks"

while the correct message would be

"MCE: warning: using only 6 banks"

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>