comms/nanovna-saver: Fixed incorrect /dev/cua* to umodem number mapping

Linux has an ioctl that directly retrieves information about the usb serial
port on (FreeBSD) I have to find the umodem dev that the usb serial port
is reference then use a sysctl to find the needed for nanovna-saver to
work correctly.
Originally I assumed a 1-1 mapping of devices. I was wrong.

PR: ports/264272
Reported by: jeffpc@josefsipek.net