net: Exclude code responsible for handling raw sockets processing

The new function - namely conn_raw_socket(); has been introduced to
handle raw sockets processing. Its code, up till now, only was
executed when IS_ENABLED(CONFIG_NET_SOCKETS_PACKET) was defined.

After this change it can be reused when one would like to handle
raw sockets also when CONFIG_NET_{UDP|TCP} are enabled.

Signed-off-by: Lukasz Majewski <lukma@denx.de>