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>