drivers: ieee802154: cc13xx_cc26xx{_subg}: fix reversed extended address

Based on the 'Technical Reference Manual' for CC13x2/CC26x2 SimpleLink
MCU family, the device contains factory pre-programmed 64-bit IEEE MAC
address for 802.15.4 radio inside two FCFG 32-bit registers:

  1. MAC_15_4_0: first 32-bit of the 64-bit IEEE MAC address
  2. MAC_15_4_1:  last 32-bit of the 64-bit IEEE MAC address

The way current version of the driver setups the address results in
incorrect bytes order (the address is reversed):

  uart:~$ ieee802154 get_ext_addr
  Extended address: AF:03:B7:25:00:4B:12:00

This fixes the problem in both drivers (also in the Sub-GHz version)
which results in use of proper EUI-64 address:

  uart:~$ ieee802154 get_ext_addr
  Extended address: 00:12:4B:00:25:B7:03:AF

IEEE MAC address was confirmed with UniFlash, nRF Sniffer for 802.15.4
and IEEE OUI database (00:12:4B is one of registered OUI for Texas
Instruments).

To prevent confusion in future, short notice about bytes order for
'mac' field in driver's data structures was also included.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>