Fix the following sparse warning:

drivers/power/supply/bd99954-charger.c:1028:6: warning: symbol 'bd9995x_chip_reset' was not declared.

The bd9995x_chip_reset() has only one call site within bd99954-charger.c
It should be static

Fixes: 0902f836 ("power: supply: Support ROHM bd99954 charger")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>