usb: gadget: Fix null pointer crash issue in gsi_free_inst

If misc register failed then there is a chance for accessing the
null pointer in gsi_free_inst function and leads to target crash.
Hence add the change to handle the same.

Change-Id: Iaa3f7db1403ca2d87f259dec109912c5437c24f4
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>