power_supply: Fix modalias for charger-manager

Since 43cc71eed1250755986da4c0f9898f9a635cb3bf, the platform
modalias is prefixed with "platform:".
Current code has the id_table, thus add MODULE_DEVICE_TABLE will
automatically setup the modalias.

Also make charger_manager_id static as it is only used in this driver.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>