usb: gps: Fix multiple gps setup issue

This patch solves a problem that happens when calling
gps_port_setup more than once. This can happen during
dynamic composition switch. In this case the wrong
port number would be assigned to port_num, which would
result in a connection error.

Change-Id: Ibdfc84c41039cb021e705e98d4cba690452c0cf8
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>