net/ieee802154: Use context's extended address when generating frame

Now that RFD is default, the extended address can be changed through net
mgmt API, and thus no longer be the same as device's generated mac address.

Change-Id: I07ee647615c2caa1994712147c6c8a2b4306900d
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>