net: bootp: Fix compile error processing ntpserver option

When the following configuration is set

  # CONFIG_CMD_DHCP is not set
  CONFIG_CMD_BOOTP=y
  CONFIG_BOOTP_NTPSERVER=y

The following compile error is observed

  error: used struct type value where scalar is required
    if (net_ntp_server)
        ^~~~~~~~~~~~~~

Resolve this by checking net_ntp_server.s_addr instead.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>