bcm43xx and ipw2100 currently duplicate the same simplistic get_stats
handler.  Additionally, zd1211rw requires the same handler to fix a
bug where all stats are reported as 0.

This patch adds a generic implementation to the ieee80211 layer,
which drivers are free to override.
Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>