mfd: Move asic3_remove to .devexit.text

The function asic3_remove is used only wrapped by __devexit_p so define
it using __devexit.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>