mrf24j40: rework tx handling to async tx handling

This patch reworks the current transmit API to spi_async handling. We
removed the error handling check, because mac802154 has no chance to
report it. Also the transmit timeout handling can't be handled by xmit
async handling, for this usecase we need to implement the netdev
watchdog. These are all unlikely cases which we drop now and should be
provided by netdev watchdog.

We also drop the bit setting for TXNACKREQ at register TXNCON, this is
not necessary. The TXNCON register should set only once for each frame,
previous settings doesn't matter.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>