hostap: Fix MAC address parsing with newlib nano

If the WPA supplicant internal event has a MAC address, and the sample
is compiled with newlib-nano then then the parsing fails because of lack
of support for C99 formatting options in nano [1].

Fix this by using supported specifier, even the code that encodes the
message uses the same format specifier.

[1] - https://docs.zephyrproject.org/latest/develop/languages/c/newlib.html#formatted-output



Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>
Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>