USB suspend/resume cannot fail so we never returned the error which
resulted in a -Wunused-but-set-variable warning.
Initialize the return variable and return a possible error possibly
triggering a printf upstream to at least have a trace of the problem.
This also fixes the warning.
Suggested by: hselasky
Sponsored by: The FreeBSD Foundation
MFC after: 10 days
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D33107
943df073a3