at86rf230: assign wait_for_completion_timeout to appropriately typed var

return type of wait_for_completion_timeout is unsigned long not int.
As rc is used here only for wait_for_completion_timeout the type is simply
changed to unsigned long.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>