Something funny happened on the way to the floppy driver today...

When two drivers share an ISA DMA channel, they both call isa_dmainit()
and the second call fails if DIAGNOSTIC is on.

If isa_dmainit() was already called successfully, just return silently.

This only works if both drivers agree on the bounce buffer size,
but since sharing DMA is usually only possible on very special
hardware and then typically only for devices of the same type (which
would have multiple instances of the same device driver), this is
not a problem in practice.

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