ASoC: sh: rz-ssi: Make return type of rz_ssi_stream_is_valid() to bool

ASoC: sh: rz-ssi: Make return type of rz_ssi_stream_is_valid() to bool

rz_ssi_stream_is_valid() never returns an int, it returns the result of
a condition which is either true or false.

While at it, drop "!!" as the expression is boolean.

Reported-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20220110094711.8574-5-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>