rtlwifi: Use order 2 RX buffer allocation only if necessary

Although a previous fix handles the kernel panics that result from
failure to allocate a new RX buffer, memory fragmentation can be
reduced if the amsdu_8k capability is disabled as new buffers need only
be of O(0), not O(2).

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>