drivers: mbox_nxp_imx_mu: return negative errno value on error

Change nxp_imx_mu_send() to return a negative errno value
on error.

The fsl_mu function MU_TriggerInterrupts() returns either
kStatus_Success or kStatus_Fail, which have the value 0
or 1, respectively. kStatus_Fail should not be returned
to the upper levels, which expect negative values for
errors, so add a check for the return value of
MU_TriggerInterrupts() and return an errno value on error.

Signed-off-by: Mike J. Chen <mjchen@google.com>