wl12xx: change debug_level module param sysfs permissions

changed the visibility of the debug_level module parameter
in the filesystem to be readable and writable to the root user.
It is now accessible under /sys/module/wl12xx/parameters

removed the debug_level debugfs file that was created under
/sys/kernel/debug/...

Signed-off-by: Guy Eilam <guy@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>