power: qpnp-qg: Update VBAT_LOW fifo_length handling

There can be a race between VBAT_LOW and FIFO_DONE irq
handler (as VBAT_LOW is triggered based on the
FIFO average). This may cause the FIFO_DONE handler
to incorrectly process the fifo-length (if the VBAT_LOW
irq handler executes just before the FIFO_DONE handler).

Fix this by updating the fifo-length only in the next
FIFO-DONE handler, after the VBAT_LOW has triggered.

Change-Id: I2829f12274692871ead9af00fc843c3bb29af4a9
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>