b43: N-PHY: implement reading support for radio 0x2057

Bit 0x200 has been noticed in the following log:
 radio_read(0x02ca) -> 0x0000
radio_write(0x00ca) <- 0x0080

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>