[PATCH] zd1211rw: Extend RF layer

These changes are needed for UW2453 RF support:

Add pointer which RF drivers can use to store private RF data
Add exit hook so that RF drivers can free private data
Allow RF's to disable the generic TX power integration handling code

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>