ieee802154: remove unused variable 'val'

Variable 'val' is not being initialized and is later being logically
or'd with DAR_PHY_CTRL4_PROMISCUOUS. Considering this variable is never
being read anyway we may as well remove val altogether.

Cleans up error detected by cppcheck:
drivers/net/ieee802154/mcr20a.c:732: (error) Uninitialized variable: val

Fixes: 8c6ad9cc ("ieee802154: Add NXP MCR20A IEEE 802.15.4 transceiver driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>