ASoC: Rename soc_enum.max field with items

The name "max" in struct soc_enum is rather confusing since it
actually takes the number of items.  With "max", one might try to
assign (nitems - 1) value.

Rename the field to a more appropriate one, "items", which is also
used in struct snd_ctl_elem_info, too.

This patch also rewrites some code like "if (x > e->nitems - 1)" with
"if (x >= e->nitems)".  Not only the latter improves the readability,
it also fixes a potential bug when e->items is zero.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>