If the device is already in a runtime PM enabled state
pm_runtime_get_sync() will return 1, so a test for negative
value should be used to check for errors.

Without this patch there are seen errors like:

[    8.540000] s3c64xx-spi 13930000.spi: Failed to enable device: 1
[    8.545000] spi_master spi1: failed to prepare transfer hardware

Likely because the driver uses synchronous API to runtime enable
the device and asynchronous one to disable it.
Signed-off-by: NSylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonielinaro.org>
Cc: stable@vger.kernel.org