hwmon: qti_amoled_ecm: return avg_current as 0 upon error exceptions

Avg_current is an unsigned 16-bit data and assigning it a negative
value would make it being shown as a very big positive value which
doesn't make sense for ECM current measurement. Instead, assign it
to 0 upon error exceptions and print out corresponding messages.

Change-Id: I438667e40d63e4142860b7de4e7cf7495d3c424e
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>