ath9k: trivial: change spectral relayfs buffering

The spectral data provided via relay-fs introduces a buffering
latency given by the subbuf_size. To meet the requirements for
delay-sensitive applications (like real-time spectral plotter),
reduce subbuf_size and increase n_subbufs.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>