drivers: ieee802154: set 'ieee802154_radio_api' as 'static const'

This change marks each instance of the 'api' as 'static const'.
The rationale is that 'api' is used for declaring internal
module interfaces and is not intended to be modified at runtime.
By using 'static const', we ensure immutability, leading to usage of only
.rodata and a reduction in the .data area.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>