drivers: ieee802154: cc13/26xx_subg: clean up constants

The CC13xx/CC26xx Sub-GHz driver header file defined several constants
that were not used in the driver.

Other constants could be replaced with generic constants which were
introduced in the prior commit.

This change removes and/or replaces redundant definitions.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>