dwc3: gadget: Initialized USB TRBs pool memory with zero

dma_coherent_alloc() API doesn't guaranteed that allocated memory
is initialized with zero. It is observed in some cases where there
is stale data seen with allocated memory for TRB pool and providing
this TRB with USB controller results into different memory access
issues. Fix this issue by using dma_zalloc_coherent() API which
allocates memory and initialized it with zero.

Change-Id: I160e319aace262958a11ba5196f53ec21b98977f
Signed-off-by: Mayank Rana <mrana@codeaurora.org>