staging: mmal-vchiq: Fix memory leak for vchi_instance

staging: mmal-vchiq: Fix memory leak for vchi_instance

The vchi_instance is allocated with vchiq_initialise() but never
handled properly. Fix memory leak for the vchi_instance.

This fixes below kmemleak report:
    [<000000002312557f>] kmem_cache_alloc_trace+0x1e0/0x348
    [<000000002ee5d470>] vchiq_initialise+0xd0/0x258
    [<000000009b51d8f4>] vchi_initialise+0x84/0xc8
    [<00000000a58f68c5>] vchiq_mmal_init+0xb0/0x2f0
    [<000000006c68d7bf>] bcm2835_mmal_probe+0x90/0x660
    ...

Change-Id: Ib10f194aa4058b3e39ec0a250fa36fd00daf9feb
Reported-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>