ASoC: fsi: tidyup: remove un-necessary operation from fsi_probe()

struct fsi_master *master became member of struct fsi_priv from
71f6e064
(ASoC: sh_fsi: avoid using global variable)

So, master = NULL is not necessary on fsi_probe() now.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>