mfd: twl6040: Fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/mfd/twl6040.c:89:20: warning:
 symbol 'twl6040_patch' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Lee Jones <lee.jones@linaro.org>