mtd: spi: Call sst_write in _write ops

Currently spi-nor code is assigning _write ops for SST
and other flashes separately. 

Just call the sst_write from generic write ops and return
if SST flash found, this way it avoids the confusion of
multiple write ops assignment during the scan and makes
it more feasible for code readability.

No functionality changes.

Cc: Simon Glass <sjg@chromium.org>
Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>