w1: ds2438: make several functions static

The functions w1_ds2438_get_page, w1_ds2438_get_temperature,
w1_ds2438_change_config_bit and w1_ds2438_get_voltage are local to
the source and do not need to be in global scope, so make it static.

Cleans up sparse warnings:
symbol 'w1_ds2438_get_page' was not declared. Should it be static?
symbol 'w1_ds2438_get_temperature' was not declared. Should it be static?
symbol 'w1_ds2438_change_config_bit' was not declared. Should it be static?
symbol 'w1_ds2438_get_voltage' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>