wl1251: set rate index and preamble flag on received packets

Set the rate index rate_idx and preamble flag RX_FLAG_SHORTPRE on received
packets.

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>