parisc: pci-dma: remove unused and dead EISA code and comment

Clearly, this code isn't needed, but it gives a false positive when
grepping the complete source tree for coherent_dma_mask.

Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
Signed-off-by: Helge Deller <deller@gmx.de>