rtl8180: make *IFS and CW tunable by mac80211, and set them in the proper place

SLOT, SIFS, DIFS, EIFS, CW and ACK-timeout registers are set in an
RF-code callback and their values are fixed.

This patch moves this off the rf-code, and introduce two new functions
that calculate these values depending by slot time and CW values
requested by mac80211.

This seems to improve performances on my setup.

Currently the ack and slot time values could be stored in a local
variable, but this patch stores it in the driver "priv" structure
because it will be useful for rtl8187se support that will be added
(hopefully) soon.

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