pcspk: convert "pit" property type from ptr to link

The speaker device needs pointer to ISA PIT device to operate. But according to
qdev-properties.h, properties of pointer type should be avoided. It seems a
link type property is a good substitution.

Signed-off-by: Efimov Vasily <real@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>