wl1271: Fix key-remove error

The new firmware does not allow removal of unicast keys - they will be
automatically removed on the next CMD_JOIN.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>