spi: nrfx_spim: fix incorrect clock control logic

To determine whether device runtime PM is enabled on a device, use
`pm_device_runtime_is_enabled`. This results in the same behaviour when
`CONFIG_PM_DEVICE_RUNTIME=n`, but properly controls the clocks on a
per-instance basis when `CONFIG_PM_DEVICE_RUNTIME=y`.

Signed-off-by: Jordan Yates <jordan@embeint.com>