leds: Use DEVICE_ATTR_{RW, RO, WO} macros

Instead of open coding DEVICE_ATTR() defines, use the
DEVICE_ATTR_RW(), DEVICE_ATTR_WO(), and DEVICE_ATTR_RO()
macros.

This required a few functions to be renamed, but the functionality
itself is unchanged.

Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>