drivers: udc_dwc2: Avoid unnecessary register accesses

Do not use sys_clear_bits() followed by sys_set_bits() on DCTL register
to avoid writing to DCTL register twice - first with zeroed out address,
and then with the new address. Change the code to write the address in
one DCTL register write.

Do not use sys_set_bits() to set test mode, but rather prepare the
correct value first.

Set DCFG and GUSBCFG registers in one go. There is no point in reading
back the value or doing multiple subsequent writes to these registers.

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