power: qpnp-smbcharger: add debug prints when usb psy is updated

Add debug prints under PR_MISC category to help verify the
communications with usb power supply.

Also take this opportunity to clean up some excessive prints in
the smbchg_external_power_changed function.

Change-Id: Id5463d42c2f40776d26d1da91106f43956e5ee91
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>