usb: dwc2: gadget: Increase descriptors count for ISOC's

Some function drivers queueing more than 128 ISOC requests at a time.
To avoid "descriptor chain full" cases, increasing descriptors count
from MAX_DMA_DESC_NUM_GENERIC to MAX_DMA_DESC_NUM_HS_ISOC for ISOC's
only.

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