iio: temperature: fix non static symbol warnings

Fixes the following sparse warnings:

drivers/iio/temperature/maxim_thermocouple.c:35:28: warning:
 symbol 'max6675_channels' was not declared. Should it be static?
drivers/iio/temperature/maxim_thermocouple.c:52:28: warning:
 symbol 'max31855_channels' was not declared. Should it be static?
drivers/iio/temperature/maxim_thermocouple.c:98:38: warning:
 symbol 'maxim_thermocouple_chips' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-By: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>