target: Fix 16-bit target ports for SET TARGET PORT GROUPS emulation

The old code did (MSB << 8) & 0xff, which always evaluates to 0.  Just use
get_unaligned_be16() so we don't have to worry about whether our open-coded
version is correct or not.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>