sf: fix sf probe

This patch fixes the "sf probe" command. The very first SPI flash probe
passes, for instance when u-boot tries to read its environment settings
from a (Q)SPI memory but next "sf probe" commands fail because the flash
memory node is unbound from the SPI controller children nodes.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Tested-by: Stefan Roese <sr@denx.de>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Simon Glass <sjg@chromium.org>