mac802154: add helper for converting dev_addr

This patch adds a helper for converting the dev_addr attribute in
netdevice to __le64 type. The dev_addr attribute is a char pointer
and contains the extended address in big endian byte order.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>