serial: make tsr_retry unsigned

It can never become negative; reflect this in the type of the field
and simplify the conditions.

Tested-by: Bret Ketchum <bcketchum@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
