iwlwifi: use Rx single frame mode (one Rx frame per RB)

The RCSR default value for single frame mode is off (a RB contains more than
one frames). But the driver assumes single frame mode apparently. This patch
enables single frame mode for the hardware. It also adds a BUG_ON to make sure
the allocated skb satisfies the hardware alignment requirement and removes
a useless check.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>