net: l2: ieee802154: shell: Validate address on input

Associate command handler did not validate the provided address length.
In result, if provided address string was longer than the expected
extended address size, strncpy() would not NULL terminate the buffer,
which could lead to unexpected behavior in parse_extended_address(), as
it expects NULL terminated string.

Fix this by validating the length of the provided address string before
parsing.

Additionally, make parse_extended_address() return the parsing result,
so that it can be detected when provided extended address has incorrect
format.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>