net/realtek-re-kmod: fix build on -CURRENT

There were several set-but-not-used variables that caused the build on
-CURRENT to fail, since this case is no longer treated as a warning
but as an error.

No port revision bump, since there is no need to rebuild existing
official packages with this patch applied. But it is required for
users that build this port on -CURRENT and 13-STABLE with the
rejection of unused variable that has been enabled there.

Approved by: portmgr (implicit)