power_supply: Fix regression for 'type' property

Commit 5f487cd3 (power_supply: Use
attribute groups) causes a regression the power supply core does not
export the 'type' attribute anymore.

POWER_SUPPLY_PROP_TYPE is handled by the power supply core without the
low-level driver, so power_supply_attr_is_visible() must always return
the entry as readable.

Reported-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Daniel Mack <daniel@caiaq.de>
Tested-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>