USB: g_file_storage: Set sense info Valid bit only when needed

Strictly speaking, the Valid bit in SCSI sense data is supposed to
be set only when the Information field contains a valid number.  This
patch (as793) turns off the Valid bit when the Information field
hasn't been set.

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