ds2781_battery: w1_ds2781_read() should be static

This patch fixes the following sparse warning:

  CHECK   drivers/power/ds2781_battery.c
drivers/power/ds2781_battery.c:72:5: warning: symbol 'w1_ds2781_read' was not declared. Should it be static?

Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>