power: qpnp-bms: improve the range of shutdown iavg

In previous revisions of the BMS peripheral, there were only 4 bits
to store the average current at shutdown. However, in the new BMS,
there are dedicated registers to store this information.

Make use of it by increasing the range and resolution of the shutdown
IAVG to 250mA + 10mA * X where X is the stored value. This will allow
for more accurate unusable charge calculations at boot time.

Change-Id: Ied413899cd05a33754c63cdc46010c941d6f083f
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>