usb: core: Simplify return value of usb_get_configuration()

It is better to initialize the return value "result" to -ENOMEM
than to 0. And because "result" takes the return value of
usb_parse_configuration() which returns 0 for success, setting
"result" to 0 at before and after of the for loop is unnecessary.

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>