ASoC: atmel_ssc_dai: enable fslen extension feature

When SSC work as master, it will generate the frame sync signal.
On old SoCs, it only supports frame sync length less or equal to
16bits, on newer SoCs, it supports frame sync length extension,
which can support frame size larger than 16 bits.
So, add this to make it supports playback 24/32 bits audio clips.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>