staging: gpib: pc2: avoid calling undefined dma_free()

staging: gpib: pc2: avoid calling undefined dma_free()

On architectures that don't support the ISA DMA API, this causes a build
failure. The corresponding dma_alloc() call is already in an #ifdef,
so use the same one for dma_free().

Note that nothing seems to set PC2_DMA, so parts of this driver
are likely unused. ISA DMA usually does not work on PCI or PCMCIA
devices, only on physical ISA slots.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20241016111521.1143191-4-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>