usb: mdm_bridge: Do not submit subsequent tx URBs in case of -EPROTO

When a write callback returns -EPROTO, treat it as if the device is
disconnected and disallow further calls to write. This will allow the
caller to gracefully handle the error until a disconnect event finally
comes and is able to notify the caller via a platform_device removal.

Change-Id: Ic780281cb5486cb5177cd642d244d2451fde1d07
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>