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