mtd: elm: Use correct check on return value of pm_runtime_get_sync

The ELM driver incorrectly reagard any non-zero return value from
pm_runtime_get_sync as an error, but it may return 1 if the device
was already active. Fix to only error when return value is negative.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>