net: openthread: radio: Added sleep to tx as hw radio cap.

OT_RADIO_CAPS_SLEEP_TO_TX was added as a radio capability
for ieee802154 radio. Waiting on RX state before transmission
is alternative condition to OT_RADIO_CAPS_SLEEP_TO_TX support
as it was a result of OpenThread architecture and is actually
not needed in the Zephyr. Such change lets to start transmission
faster and lower SED device power consumption in active state
about 30%.

Signed-off-by: Kamil Kasperczyk <kamil.kasperczyk@nordicsemi.no>