mtd: m25p80: add debugging trace in sst_write

Add a DEBUG(MTD_DEBUG_LEVEL2, ..) trace at beginning of sst_write() function as
it is done in m25p80_write() function.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>