Revert "drivers: usb: usb_dc_nrfx: Set cfg.en to false on Reset"

This reverts commit 03ef375f.

The cfg.en flag contains only the state of the endpoint as seen by the
shim. It is mainly used to enable the endpoints when the USB peripheral
becomes ready for operation (and the USB stack may want to enable some
endpoints earlier, especially the control ones, so the operation must
be sometimes deferred). In particular, setting this flag to false has
no effect on the actual state of the endpoint in the hardware.
Moreover, this flag was set to false for all the endpoints, including
the control ones which should not be disabled, so such operation
actually fooled the shim.

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