drivers: udc_dwc2: Respect maximum fifo depth

When dynamic fifo sizing is enabled the driver can assign fifo size
lower or equal to the reset value. Software must respect the maximum
sizes because fifo width registers are optimized during synthesis and
the larger values simply get discarded.

Use 16-bit values for FIFO depths because maximum FIFO depth the vendor
can configure the DWC2 otg is 32768.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>