mailbox: imx: fix crash in resume on i.mx8ulp

check 'priv->clk' before 'imx_mu_read()' otherwise crash happens on
i.mx8ulp, since clock not enabled.

Fixes: 4f0b776e ("mailbox: imx-mailbox: support i.MX8ULP MU")
Reviewed-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>