of: Fix __of_device_is_available check

From IEEE 1275, there defined a standard 'status' property indicating the
operational status of one device. The 'status' property has four possible
values: 'okay/ok', 'disabled', 'fail' and 'fail-xxx'.

If it is absent, that means the status of the device is unknown or okay.

The __of_device_is_available checks the state of the 'status' property of
a device. If the property is absent or set to 'okay/ok', it returns 1.
Otherwise it returns 0.

While in __of_device_is_available:
 >	status = of_get_property(device, "status", &statlen);
 >	if (status == NULL)
 >		return 1;
The status value returned from 'of_get_property()' will be NULL in two cases:
Firstly: the 'device' value (device node) is NULL.
Secondly: the 'status' property is actaully not exist.

If the device node is NULL, the __of_device_is_available will return true,
that will mean the absent state of the 'status' property.

So this add the device node check before checking the 'status' property's
state, and if the device node is not exist, 0 will be returned.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>