sf: Enable FSR polling on N25Q256(A)

The N25Q256(A) datasheet clearly states that this device does have
a Flag Status Register and does update FSR PEC bit 7 during Program
and Erase cycles to indicate the cycle is in progress. Enable the
FSR PEC bit polling on this device to prevent data corruption.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@openedev.com>
Cc: Tom Rini <trini@konsulko.com>