[SPARC64]: Fix section error in sparcspkr

With a sparc64 defconfig modified to set CONFIG_HOTPLUG=n
the following error happened during link of vmlinux:

local symbol 0: discarded in section `.devexit.text' from drivers/built-in.o
local symbol 1: discarded in section `.devexit.text' from drivers/built-in.o

(The error message above is from kbuild.git but it happens in mainline too)

The error happens becase there is a reference from .text/.data to a
function marked __devexit. With CONFIG_HOTPLUG=n all code marked
__devexit are discarded and the linker complains.

It was tracked down to sparcspkr.c which were missing __devexit_p()
around the function pointers.

Unfortunately modpost did not catch this since modpost do not warn
about references from .data to .devexit from variables named *_driver.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>