pcf50633: Query charger status directly

Current scheme is fragile and is likely to go off sync, especially on
batfull->adapter charging automatic MBC transition.

Query the status bit every time we need it instead.

We need to export another function to query for USB presence because
we can't read anything from PCF50633 (via I2C) inside irq context and
that is needed by usb gadgets.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>