ASoC: Correct typo in SOC_VALUE_ENUM_SINGLE macro

xnitmes is clearly intended to be xnitems, but all other macros just
refer to this as xitems, so change it to that.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>