staging: rtl8712: Replace function r8712_dequeue_cmd()

Remove function r8712_dequeue_cmd as all it does is call _dequeue_cmd.
Rename _dequeue_cmd to r8712_dequeue_cmd and change its type from static
to non-static to maintain compatibility with call sites.
Issue found with Coccinelle.

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