power: qpnp-charger: use is_usb/dc_chg_plugged in

Remove code that doesn't check for null pointer exceptions
and instead use the safe internal function to check wether
a USB or DC charger is connected.

Change-Id: I80cba398b0928dc8d15cdb6f8af6c49e515448ef
Signed-off-by: David Keitel <dkeitel@codeaurora.org>