usb: dwc3: increase maximum number of TRBs per endpoint

previously we were using a maximum of 32 TRBs per
endpoint. With each TRB being 16 bytes long, we were
using 512 bytes of memory for each endpoint.

However, SLAB/SLUB will always allocate PAGE_SIZE
chunks. In order to better utilize the memory we
allocate and to allow deeper queues for gadgets
which would benefit from it (g_ether comes to mind),
let's increase the maximum to 256 TRBs which rounds
up to 4096 bytes for each endpoint.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>