usb: gadget: f_rmnet: Make sure to have ep->ops updated before connect

msm_ep_config() overrides ep->ops for USB endless queue functionality.
There is possible race when ep->ops are being used from
gbam2bam_connect_work() and as same time it is being updated from
frmnet_set_alt(). Hence make sure to update ep->ops before calling
gbam_connect() API to queue endless transfer with USB endpoint.

Also fix error handling with frmnet_set_alt() API.

Change-Id: I6f51b0f0ae9f79d1a19b6173e7f65be2dca71404
Signed-off-by: Mayank Rana <mrana@codeaurora.org>