usb: core: Move variable initialization to appropriate place

It is better to initialize the variable 'cfgno' in the for loop than
at the current place.

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>