usb: storage: Only clear READY flag on error

Clear the USB_READY flag in the storage driver only in case there
is an error, otherwise usb_stor_BBB_transport() waits 5 mS before
doing anything every single time.

This is because the USB_READY flag is only ever set in
usb_test_unit_ready(), which is called only upon storage device
probe, not between each and every request. However, the device
cannot move out of USB_READY state once it was initialized.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>