net: if: Fix potential unaligned word access

The input address originates from an RX net_buf which may
be unaligned. Consequently, word access to this unaligned buffer
can trigger an unaligned access exception on certain platforms,
such as the ARMv7 Cortex-A9.

Signed-off-by: Taras Zaporozhets <zaporozhets.taras@gmail.com>