northbridge/amd/gx2,lx: Treat MSR constant as unsigned long

Clang complains that a signed shift result (0x210000000)
requires 35 bits to represent, but 'int' only has 32 bits.
However, we write the high bits separately and so this is
a spurious warning.

Change-Id: I3e1c57334077feb50004d7b39abff4bd84ca095b
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7673
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)