Bluetooth: bnep: Fix out-of-bound access

This fixes attempting to access past ethhdr.h_source, although it seems
intentional to copy also the contents of h_proto this triggers
out-of-bound access problems with the likes of static analyzer, so this
instead just copy ETH_ALEN and then proceed to use put_unaligned to copy
h_proto separetely.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>