power: qpnp-smbcharger: fix null pointer access in parallel chg

The handle_usb_insertion()/removal()  do not check if the
parallel_psy pointer is null before accessing it.
Fix it.

Change-Id: I15e88841eaa7a1fec611c82403e6dfa444d8cb9a
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>