net: ethernet: bgmac: Remove unnecessary 'return' from platform_bgmac_idm_write

Return type for idm register write callback should be void as 'writel'
API is used for write operation. However, there no need to have 'return'
in this function.

Signed-off-by: Abhishek Shah <abhishek.shah@broadcom.com>
Reviewed-by: Oza Oza <oza.oza@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>