ASoC: max98088: Staticise m98088_eq_band

This function is not exported and it does not seem to be called from
anywhere else therefore it should be static.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>