mgmt: hawkbit: remove NET_SOCKETS_POSIX_NAMES dependency

Currently, it is not possible to use hawkbit with code that requires
POSIX_API to be set due to the dependency on NET_SOCKETS_POSIX_NAMES.
Since a lot of other code has already been moved to `zsock_`, this
commit does the same for hawkbit.

Co-authored-by: rojedag <r.ojeda@vogl-electronic.com>
Signed-off-by: Matthias Breithaupt <m.breithaupt@vogl-electronic.com>