southbridge/amd/rsXY0/cmn.c: Fix bitwise logic and mask in loop

Correct mask to select bits 4-6 inclusively as per comment and use
bitwise operations while working with bits. Be sure to write back out
the data on the retrain.

Change-Id: I26e7acddbff32e978c2bf984c21d9a63337067f8
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Found-by: Clang
Reviewed-on: http://review.coreboot.org/6147
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Tested-by: build bot (Jenkins)