power: supply: cpcap-charger: cpcap_charger_voltage_to_regval() can be static

Add 'static' keyword to internal function as reported by sparse
using the following configuration:

make ARCH=x86_64 allmodconfig
make C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__'

Fixes: d4ee021c ("power: supply: cpcap-charger: Limit voltage to 4.2V for battery")
Signed-off-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>