iio: dac: set variable max5522_channels storage-class-specifier to static

smatch reports
drivers/iio/dac/max5522.c:55:28: warning: symbol
  'max5522_channels' was not declared. Should it be static?

This variable is only used in one file so it should be static.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20230404013828.1914523-1-trix@redhat.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>