drivers: usb_dc_nrfx: Correct the nrfx_power driver configuration

The initial configuration provided for the nrfx_power driver cannot
contain just default values, as that mean that DC/DC converters are
to be disabled, while those converters may be (and they actually are
by default for many boards) enabled through Kconfig.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>