power: qpnp-fg: handle battery missing irq in threaded interrupt handler

Currently battery missing interrupt is handled in an interrupt
handler. There are some situations where it can sleep. Hence
move it to a threaded interrupt handler to handle that.

While at it, fix the error return code handling when the
interrupt registration fails.

CRs-Fixed: 837984
Change-Id: I5a79f24d809809cbef7b8b93e9942b72b1bfc335
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>