net: usb: rmnet_usb_ctrl: Fix return value of rmnet_ctl_write()

When mux is enabled value returned by rmnet_ctl_write() also
includes mux header size. Hence fix this by returning correct
size.

Change-Id: Iba4eda7604109841c5bc6aeda59d9f02a9927c21
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>