net: utils: Byte to hex converter had wrong prototype

The net_byte_to_hex() prototype first parameter was uint8_t *,
but it should have been char *.

Jira: ZEP-1885

Change-Id: I6132a67bb9e8199de88451fb4e446081f401e8f6
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>