libertas: store rssi as an u32

Don't store an (hardware base) u8 value in bss_descriptor, but just an
unsigned int (RSSI is really unsigned). Compilers generate more efficent
code for ints than for bytes.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>