leds: turris-omnia: convert to use dev_groups

The driver core supports the ability to handle the creation and removal
of device-specific sysfs files in a race-free manner.  Take advantage of
that by converting this driver to use this by moving the sysfs
attributes into a group and assigning the dev_groups pointer to it.

Cc: Pavel Machek <pavel@ucw.cz>
Cc: linux-leds@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>