rtl8180: report mac80211 short preamble RX flag

This patch make it possible to mac80211 to know whether a frame
has been received with short preamble.

It simply checks for the "splcp" flag in the RX status
descriptor, and eventually set RX_FLAG_SHORTPRE in mac80211
rx status structure.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>