net/shell: Simplify ping logic

net_addr_pton always returns -EINVAL in case of error, so let's play with
it to factorize the code.

net_icmpv<4/6>_send_echo_request always returns -EIO in case of error.

Change-Id: I89e5ccd4b936701f7dad194089dda845fab5d738
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>