[PATCH] zd1211rw: rework band edge patching

This change allows RF drivers to provide their own 6M band edge patching
implementation, while providing a generic implementation shared by most
currently supported RF's.

The upcoming ZD1211B/AL7230B code will use this to define its own
patching function, which is different from the other RF configurations.

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