rndis_wlan: simplify is_associated()

It is not necessary to define the variable ret to receive
the return value of the get_bssid() method.

Signed-off-by: zuoqilin <zuoqilin@yulong.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210423094940.1593-1-zuoqilin1@163.com