staging: rtl8712: Change (r8712)_enqueue_cmd to void

Change the return types of r8712_enqueue_cmd to void as the return value
of r8712_enqueue_cmd is never stored, checked, or otherwise used.
Also change the return type of _enqueue_cmd to void as it is only called
by r8712_enqueue_cmd which does not do anything with the return value
except return it itself.
Modify return statements in both functions accordingly.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>