usb: gadget: gsi: Decrement USB gadget pm usage count on cable disconnect

Currently USB GSI function driver increments USB gadget device's
pm usage count on set_alt() and decrements on USB bus suspend or
USB cable disconnect case. Current code is not decrementing this
usage count when sm_state is STATE_INITIALIZED and USB cable is
disconnected (i.e. event EVT_DISCONNECTED posted). Fix this issue
by decrementing USB gadget device's pm usage count by addding check
for event EVT_DISCONNECTED when sm_state is STATE_INITIALIZED.

CRs-Fixed: 1003242
Change-Id: I4d6c9ce254f4c4139313dfd33da6c8745f34a1d3
Signed-off-by: Mayank Rana <mrana@codeaurora.org>