Check for errors when detaching children first, not last

These detach routines in these drivers all ended with 'return
(bus_generic_detach())' meaning that if any child device failed to
detach, the parent driver was left in a mostly destroyed state, but
still marked attached. Instead, bus drivers should detach child
drivers first and return errors before destroying driver state in the
parent.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D47387

(cherry picked from commit d412c07617eb35435668b024bc2cecda05f57f1f)