cam_periph: fix bug in camperiphunitnext logic

If we assigned just a lun as a wired unit (something that camperiphunit
will accept), we failed to properly skip over that unit when computing a
next unit number. Add lun so the code matches the comments that we have
to skip all the same criteria that camperiphunit uses to select wired
units for a driver.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D32682