usb: storage: scsiglue: further describe our 240 sector limit

Just so we have some sort of documentation as to why
we limit our Mass Storage transfers to 240 sectors,
let's update the comment to make clearer that
devices were found that would choke with larger
transfers.

While at that, also make sure to clarify that other
operating systems have similar, albeit different,
limits on mass storage transfers.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>