drivers: usb: nordic: Fix for startup with USB cable attached.

Fixes bug introduced in #14875.
USBDETECTED event is be generated on cable attachment and
when cable is already attached during reset, but not when
the peripheral is re-initialized. When USB-enabled bootloader
is used, target application will not receive this event
and it needs to be generated again. This commit implements
a check against driver being enabled multiple times in case
of application without bootlader starting with cable attached
(both "fake" and "real" events are generated). Such dirty
trick allows bootloader to leave the peripheral in any state
before jumping to target application.

Fixes #15073

Signed-off-by: Paweł Zadrożniak <pawel.zadrozniak@nordicsemi.no>