sf: Fix NULL pointer exception for flashes without lock methods

commit c3c016cf "sf: Add SPI NOR protection mechanism" introduced
flash_lock()/flash_unlock()/flash_is_locked() methods for SPI flash,
but not every flash driver supplies these. We should test these
methods against NULL before actually calling them.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>