libertas: remove some references to IW_MODE_abc

... in pursue to quaff the wide-spread references to WEXT constants.

When setting SNMP_MIB_OID_BSS_TYPE, wext.c can directly calculate the value
the firmware wants.

Reading of SNMP_MIB_OID_BSS_TYPE doesn't happen anywhere, so no need to
convert the firmware value into WEXT values anyway.

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