usb: gadget: f_gsi: Check USB connection status in ioctl

In case of mdm subsystem re-start when usb is in disconnect
state, qti issues QTI_CTRL_MODEM_OFFLINE ioctl. This is
resulting into config NULL pointer dereference to access
usb composite device struct. Fix this issue by adding a
check for cable connection status before handling any ioctl.
Also move access of cdev pointer after usb connection status
check in gsi_ctrl_send_notification().

Change-Id: I8e62b69cfa4da0300d02d4bf6ddaf445f4577ba5
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>