spmi: qpnp-int: add irq_ack() callback and use it inside irq_mask_ack()

Add a function into the qpnp-int driver named qpnpint_irq_ack()
which performs the single step required to acknowledge a PMIC
interrupt: clearing the latched status register bit for the
interrupt.  At the same time modify the qpnpint_irq_mask_ack()
function so that it simply calls qpnpint_irq_mask() and
qpnpint_irq_ack().  This eliminates redundant code present in
both qpnpint_irq_mask() and qpnpint_irq_mask_ack().

An irq_ack() callback is required in order to utilize the
handle_edge_irq flow handler.

Change-Id: Iacb0edf01fe82f358bb6200cef1b905584eb29e5
Signed-off-by: David Collins <collinsd@codeaurora.org>