pbio: Store softc in si_drv1 for character devices.

The port number is still stored in the unit (si_drv0) but is the
entire unit value now.

While here, removed checks for NULL softc since those can never happen
from cdevsw routines. This also resulted in the close method becoming
a no-op, so it has been removed.

Differential Revision: https://reviews.freebsd.org/D35078