mailbox: bcm-pdc: Remove unnecessary void* casts

Remove unnecessary void* casts in register writes. Fix two other
minor formatting issues.

Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Reviewed-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>