thunderbolt: Fix property read in nhi_wake_supported()

device_property_read_foo() returns 0 on success and only then modifies
'val'. Currently, val is left uninitialized if the aforementioned
function returns non-zero, making nhi_wake_supported() return true
almost always (random != 0) if the property is not present in device
firmware.

Invert the check to make it make sense.

Fixes: 3cdb9446a117 ("thunderbolt: Add support for Intel Ice Lake")
Cc: stable@vger.kernel.org
Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>