mac802154: correct max sifs size handling

This patch fix the max sifs size correction when the
IEEE802154_HW_TX_OMIT_CKSUM flag is set. With this flag the sk_buff
doesn't contain the CRC, because the transceiver will add the CRC
while transmit.

Also add some defines for the max sifs frame size value and frame check
sequence according to 802.15.4 standard.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>