usb: f_gsi: Simplify handling of remote_wakeup timer

Driver uses a timer to initiate remote-wakeup after
USB bus gets suspended (U3 or function suspend state).
This timer is currently enabled by user using debugfs.
Simplify handling of this timer and also have a generic
name for timer_fn so that it can be invoked from elsewhere
as well.

Change-Id: Ic535aa2ef84c864bf59919631c438df4f1249a11
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>