serial: simplify tsr_retry reset

Move common code outside the if, and reset tsr_retry even in loopback mode.
Right now it cannot become non-zero, but it will be possible as soon as
we start respecting the baud rate.

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