pm8xxx: Remove __exit marking on pm8xxx_irq_exit()

This function is called from more places than just exit
functions. Also remove the _init/__exit markings from the header
file as they're not necessary in function prototypes.

WARNING: vmlinux.o(.devinit.text+0x7840): Section mismatch in
reference from the function pm8038_add_subdevices() to the
function .devexit.text:pm8xxx_irq_exit()
The function __devinit pm8038_add_subdevices() references
a function __devexit pm8xxx_irq_exit().
This is often seen when error handling in the init function
uses functionality in the exit path.
The fix is often to remove the __devexit annotation of
pm8xxx_irq_exit() so it may be used outside an exit section.

Change-Id: I0d2e285a03958e0fe68af48df94523aa1d0476c0
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>