hwmon: (w83781d) Fix linking when built-in

When w83781d is built-in, the final links fails with the following vague error
message:

`.exit.text' referenced in section `.init.text' of drivers/built-in.o: defined
in discarded section `.exit.text' of drivers/built-in.o

w83781d_isa_unregister() cannot be marked __exit, as it's also called from
sensors_w83781d_init(), which is marked __init.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>