mtd: m25p80: don't probe device which has status of 'disabled'

On some platforms such as P3060QDS, has multiple spi flashes, but they are
not available at same time, so if their status is 'disabled', which is set
by u-boot, will not be probed.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>