USB storage probe

While debugging one ill behaving USB device I found two bugs in USB
storage probe.

usb_stor_get_info() returns -1 (error), 0 (skip) or 1 (ok). First part
of this patch fixes error case.

Second part fixes usb_inquiry()'s retry counter handling. Original code
had retry = -1 on error case, not retry = 0 as checked in the next line.

Signed-off-by: Kim B. Heino <Kim.Heino@bluegiga.com>