power: qpnp-smb2: use correct properties

CURRENT_MAX and VOLTAGE_MAX should be used to show the current values
for the charger. The driver currently uses them to communicate with
the USB and PD drivers.

Update the code such that USB uses SDP_CURRENT_MAX to set 500mA/900mA
current limits after enumeration and PD uses PD_VOLTAGE_MIN/MAX to set
the adapter voltage limits.

CURRENT_MAX now shows the max current that can be drawn from the charger
and VOLTAGE_MAX shows the max voltage that the charger can negotiate up
to.

CURRENT_MAX and VOLTAGE_MAX can no longer be set.

Change-Id: I660e39effc1862051cff8b9babc885d8de3f9c84
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>