NAND: Silence warning when CONFIG_SYS_NAND_QUIET_TEST

Commit cfa460ad removed support
for disabling the "No NAND device found!!!" warning when
CONFIG_SYS_NAND_QUIET_TEST was defined.  This re-adds support
for silencing the warning.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>