drivers: udc_dwc2: fix interpretation of NUMDEVEPS and INEPS fields

The NUMDEVEPS field provides the number of endpoints in addition to the
control endpoint. It is used to iterate over GHWCFG1 register value to
get correct number of configured IN/OUT endpoints. To get it correctly,
we need to use it internally as number including control endpoint.

Interpretation of INEPS misses +1 because value 0 means 1 IN endpoint
and so on.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>