orinoco: encapsulate driver locking

Local bus and USB drivers will need to do locking differently.

The original orinoco_usb patches had a boolean variable controlling
whether spin_lock_bh was used, or irq based locking. This version
provides wrappers for the lock functions and the drivers specify the
functions pointers needed.

This will introduce a performance penalty, but I'm not expecting it to
be noticable.

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>