USB: gadget: Fix port allocation issue on next USB cable connect

commit "457ac5b0 USB: gadget: Fix race condition to over come memory
corruption" fixed memory corruption issue with dynamic composition
switch for port structure. Although it misses handling of USB cable
disconnect/connect part and due to which USB composition having
RNDIS/ECM/MBIM composition is failing on USB cable disconnect/connect
case. This change accomodates both dynamic composition switch and
USB cable disconnect/connect usecases.

CRs-Fixed: 653163
Change-Id: I2880e6f534bbc7bdcd92692acf5c56397446401e
Signed-off-by: Mayank Rana <mrana@codeaurora.org>