mainboard/amd: Incorrect usage of logical vs. bitwise and

Spotted by Clang

Change-Id: I26201c7f5e421c38d3965d8e7e62c4a8e670e449
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5833
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martin.roth@se-eng.com>