b43: Handle B43_PHYTYPE_LP in RX path

Don't drop all packets received from an LP-PHY with WARN_ON.
Also update a comment with LP-specific information.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>