brcm80211: drop unreachable else case

the if/elseif/else is exhaustive - there is no 4th case given the
  rssi_ctrl_mask = RADIO_2055_NBRSSI_SEL | RADIO_2055_WBRSSI_G1_SEL |
  RADIO_2055_WBRSSI_G2_SEL;
so this unreachable else case (dead code) can be dropped.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>