usb: gadget: f_mbim: Remove usage of notify_state

Driver can rely on device->online attribute to
figure out whether interrupt notification must be
sent or not instead of separate notify_state enum.

Change-Id: I82aa5c18363f30d7d40c820858a1396dca86a07e
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>