dwc2: gadget: Fix ISOC IN DDMA PID bitfield value calculation

PID bitfield in descriptor should be set based on particular request
length, not based on EP's mc value. PID value can't be set to 0 even
request length is 0.

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