USB: usb-storage: don't access beyond the end of the sg buffer

This patch (as1035) fixes a bug in usb_stor_access_xfer_buf() (the bug
was originally found by Boaz Harrosh): The routine must not attempt to
write beyond the end of a scatter-gather list or beyond the number of
bytes requested.  It also fixes up the formatting of a few comments
and similar whitespace issues.

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