staging: spectra: don't read past array in Conv_Spare_Data_Log2Phy_Format()

It should decrement or we read past the array

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>