Can: mcan: Uninitialized access in state_change_handler

Found via static analysis. In `can_mcan_state_change_handler`, the
return value of `can_mcan_get_state` is not checked for error. In the
event of an error, both `err_cnt` and `state` are left unitialized and
accessed downstream.

The correct behavior is to return early in that case.

Signed-off-by: François Baldassari <francois@memfault.com>