regulator: rk808: Fix sparse non static symbol warnings

Fixes the following sparse warnings:

drivers/regulator/rk808-regulator.c:100:5: warning:
 symbol 'rk808_set_suspend_voltage' was not declared. Should it be static?
drivers/regulator/rk808-regulator.c:115:5: warning:
 symbol 'rk808_set_suspend_enable' was not declared. Should it be static?
drivers/regulator/rk808-regulator.c:126:5: warning:
 symbol 'rk808_set_suspend_disable' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mark Brown <broonie@kernel.org>