add check operation to struct mx_ops

add check operation to struct mx_ops

In mx_check_mailbox switch case, we simply call
<mailbox>_check_mailbox, so this operation can be move into the mx_ops
structure pretty easily.

This commit adds a mandatory "check" operation to struct mx_ops and
change all mailboxes to use it. Check functions are made static as they
are only used in their respective source files now.