pm2301-charger: Staticize pm2xxx_charger_die_therm_mngt

pm2xxx_charger_die_therm_mngt is used only in this file. Make it static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>