ab8500_fg.c: only request threaded IRQs when necessary

ab8500_fg.c: only request threaded IRQs when necessary

All 5 IRQ handlers of the driver are requested as threaded interrupt
handlers.  However, only 1 handler can block.  The remaining 4 handlers
defer the actual handling to a workqueue.  Hence, 4 of 5 IRQ handlers
have a considerable overhead, since they are executed in a kernel thread
to schedule another kernel thread (workqueue).

This change splits up the 5 interrupt handlers into top halves (_th) and
bottom halves (_bh) and resolves the aforementioned overhead by only
requesting threaded interrupts (i.e., bottom halves) when necessary.

Signed-off-by: Valentin Rothberg <Valentin.Rothberg@lip6.fr>
Signed-off-by: Sebastian Reichel <sre@kernel.org>