To avoid duplicate code, use ieee80211_[pspoll|nullfunc]_get() to get
templates for PS Poll and Nullfunc frames in mlme.c.

Compile-tested only.
Signed-off-by: NKalle Valo <kalle.valo@nokia.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>