ide-dma: fix ide_build_dmatable() for TRM290

Apparently, 'xcount' being 0 does not mean 0 bytes for TRM290; it means 4 bytes,
judging from the code immediately preceding this check.  So, we must never try
to "split" the PRD for TRM290.

This is probably never hit anyway -- with the DMA buffers aligned to at least
512 bytes and ATAPI DMA not being used for non block I/O commands...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>