staging: rtl8712: Replace function r8712_enqueue_cmd()

Merge function _enqueue_cmd into function r8712_enqueue_cmd as
_enqueue_cmd is only called by r8712_enqueue_cmd, and then only once.

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