net: correct in*_addr parameter of net_addr_pton()

Currently, the function accepts a struct sockaddr * but the code
immediately type casts this to either in_addr or in6_addr.  This is
incorrect behavior as the first field in a sockaddr is sa_family_t
and not address data.

So without special knowledge, a developer will use a sockaddr structure
as the parameter and then wonder why the address information isn't being
set correctly.

Let's change this parameter to void * which makes this function similar
to inet_pton().

Jira: ZEP-1616

Change-Id: I1fc9368da999d90feb07c03fac55dcc749d4eba6
Signed-off-by: Michael Scott <michael.scott@linaro.org>