usb: dwc2: gadget: pick smallest acceptable fifo

Current algorithm picks the first fifo which is equal to or greater
than the required size. This can result in bigger fifos assigned to
endpoints with smaller maxps. Change the algorithm to pick the
smallest fifo which is greater than or equal to the required size.

Moreover, only use signed variables when required.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>