usb: mdm_bridge: always read interrupt notifications

The MDM device connected over HSIC may generate interrupts containing
CDC notifications. Currently, however, the URBs to read the interrupt
endpoint are not submitted unless the bridge is opened, i.e. USB
cable is connected. Thus, the interrupt IN would remain unread, and
if the HSIC bus attempts to enter suspend the MDM now continues to
generate remote wakeup events preventing low power mode.

This change simply allows submission of the interrupt URB as soon as
the bridge driver successfully probes. The contents of the notification
are stored anyway, so notify the client when it finally opens the bridge.

CRs-Fixed: 365015
Change-Id: I3a628be34c62fef1388d02d6adf052f2e4c77ac2
Signed-off-by: Jack Pham <jackp@codeaurora.org>