USB: In init_endpoint_class, use PTR_ERR to obtain an errno value, not IS_ERR

init_endpoint_class calls class_create, and checks the result for an error
with IS_ERR; however, if true, it then returns the result of IS_ERR (a
boolean) rather than PTR_ERR (the actual errno).

Signed-off-by: Josh Triplett <josh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>