usb-storage: update earlier scatter-gather bug fix

usb-storage: update earlier scatter-gather bug fix

This patch (as1037) makes a small update to the earlier as1035 patch.
The minimum-length computation shouldn't be done in
usb_stor_access_xfer_buf(), since that routine can be called multiple
times for a single transfer.  It should be done in
usb_stor_set_xfer_buf() instead, which gets called only once.

The way it is now isn't really _wrong_, but it isn't really _right_
either.  Moving the statement will be an improvement.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>