As a follow-up to a97bda7d, report the
external sample rate as system_sample_rate when in slave mode.

For PCIe MADI cards, the DDS value automatically contains the external
sample rate, but the PCI version needs this manual workaround.
Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>