mtd: spi-nor: fix spansion quad enable

With the S25FL127S nor flash part, each writing to the configuration
register takes hundreds of ms. During that  time, no more accesses to
the flash should be done (even reads).

This commit adds a wait loop after the register writing until the flash
finishes its work.

This issue could make rootfs mounting fail when the latter was done too
much closely to this quad enable bit setting step. And in this case, a
driver as UBIFS may try to recover the filesystem and may broke it
completely.

Signed-off-by: Joël Esponde <joel.esponde@honeywell.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>