b43: fix b43_plcp_get_bitrate_idx_ofdm return type

This patch fixes the return type of b43_plcp_get_bitrate_idx_ofdm. If
the plcp contains an error, the function return value is 255 instead
of -1, and the packet was not dropped. This causes a warning in
__ieee80211_rx function because rate idx is out of range.

Cc: stable@kernel.org
Signed-off-by: Lorenzo Nava <navalorenx@gmail.com>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>