staging: rtl8723bs: Replace string with identifier

Replace the hardcoded function names with the corresponding
predefined identifiers.
Issue found by checkpatch.

Signed-off-by: Javier F. Arias <jarias.linux@gmail.com>
Link: https://lore.kernel.org/r/b2ff6a52a7140480c9fdb6486a6ac6b4eb51203b.1571284318.git.jarias.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>