staging/usbip: fix store_attach() sscanf return value check

sscanf() parses the input buffer for four input items. However,
the return value check is incorrect, as it checks for one input
item instead of four which is what it is expecting in the input
buffer. As a result, sscanf() will always fail even when the input
buffer is correct.

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>