USB: android: Allow reducing number of serial interfaces on comp switch

serial_function_bind_config will do initialization only once
and will only do bind for next composition switches with this we
can not support decrease or increase in number of serial
interfaces.

However it is possible to support compositions with number of
serial interfaces less than initialized. This change adds code
to handle the case where number of serial interface is less than
initialized. With this we do not support increasing number of
serial interfaces.

Change-Id: I2820e17199d94d898eaa367b63e6836f78d14d3e
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>