ieee802154: mcr20a: Replace magic number with constants

The combination of defined constants are used to present the
state of IRQ so the magic numbers has been replaced.

This is a simple coding style change which should have no impact on
runtime code execution.

Signed-off-by: Xue Liu <liuxuenetmail@gmail.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>