testusb: remove all mentions of 'usbfs'

Commit 8a424bf4 (tools/usb: remove last USBFS
user) removed 'usbfs' files from the source but retained mentions of 'usbfs'
all over the place, most importantly in the misleading error messages printed
in case USB device files are not there.  Remove all the  mentions of 'usbfs'
for good now!

Signed-off-by: Sergei Shtylyov <sshtylyov@dev.rtsoft.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>