usb: gadget: Fix of Android next_string_id.

Remove next_string_id resetting. This field was already updated in probe
time when calling the android_bind function for the manufacturer id and
for the product id. Thus could not be reset in the store_enable function
that is called after, and override those string ids.

Change-Id: I2cf84ec7252dea0701927fab31f6ec1feb35d63f
Signed-off-by: Ofir Cohen <ofirc@codeaurora.org>