mwl8: fix build warning.

This patch fixes a build warning in mwl8.c.
(Marvell TOPDOG wireless driver)

The warning it fixes is: "large integer implicitly truncated to unsigned type."

The rx_ctrl member of the mwl8k_rx_desc struct is 8 bit (__u8 ), whereas trying
to assign it a 32 bit value (which is returned from  cpu_to_le32())
causes the compiler to issue
a truncation warning.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>