mfd: twl6030: Make twl6030_exit_irq() return void

This function returns 0 unconditionally, so there is no benefit in
returning a value at all and make the caller do error checking.

Also the caller (twl_remove()) cannot do anything sensible with an error
code. Passing it up the call stack isn't a good option because the i2c core
ignores error codes (apart from emitting an error message).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220113101430.12869-2-u.kleine-koenig@pengutronix.de