PM / Domains: Remove dev->driver check for runtime PM

Remove check for driver of a device, for runtime PM. Device may be
suspended without an explicit driver. This check seems to be vestigial
and incorrect in the current context.

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Lina Iyer <lina.iyer@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>