drivers: change struct device_driver::coredump() return type to void

Upon submitting a patch for mwifiex [1] it was discussed whether this
callback function could fail. To keep things simple there is no need
for the error code so the driver can do the task synchronous or not
without worries. Currently the device driver core already ignores the
return value so changing it to void.

[1] https://patchwork.kernel.org/patch/10231933/



Signed-off-by: Arend van Spriel <aspriel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>