usb: function: f_gsi: Allow line state ioctl query

As part of gsi_disable we send zero length packet to the
user space daemons(i.e. QTI/QBI). These daemons when they
receive a ZLP, query for line state to manage the open/close
of their DPM ports. Allow the line state ioctl query irrespective
of gsi connected state.

Change-Id: I4cd50c2b9273ffdeff561dbb1e4cd248cea2139b
Signed-off-by: Elson Roy Serrao <eserrao@codeaurora.org>