staging: vt6656: fix sign of rx_dbm to bb_pre_ed_rssi.

commit 93134df5 upstream.

bb_pre_ed_rssi is an u8 rx_dm always returns negative signed
values add minus operator to always yield positive.

fixes issue where rx sensitivity is always set to maximum because
the unsigned numbers were always greater then 100.

Fixes: 63b9907f

 ("staging: vt6656: mac80211 conversion: create rx function.")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/aceac98c-6e69-3ce1-dfec-2bf27b980221@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>