mailbox: imx: remove redundant initializer

Smatch reports this issue
imx-mailbox.c:887:10: warning: Initializer entry defined twice
imx-mailbox.c:889:10:   also defined here

  .rxdb	= imx_mu_generic_rxdb,

Is listed twice, so remove one.
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/imx-mailbox.c

@@ -886,7 +886,6 @@ static const struct imx_mu_dcfg imx_mu_cfg_imx8ulp = {
 	.rx	= imx_mu_generic_rx,
 	.rxdb	= imx_mu_generic_rxdb,
 	.init	= imx_mu_init_generic,
-	.rxdb	= imx_mu_generic_rxdb,
 	.type	= IMX_MU_V2,
 	.xTR	= 0x200,
 	.xRR	= 0x280,