net/niu: remove one superfluous dma mask check

The idea here seems to be to get a 44bit DMA mask working and if this
fails it should fallback to a 32bit DMA mask. The dma_mask variable is
assigned once to 44bit and never updated. pci_set_dma_mask() and
pci_set_consistent_dma_mask() are both implemented as functions so there
is no evil macro which might update dma_mask. Looking at the assembly, I
see a call to dma_set_mask() followed by dma_supported() and then a jump
passed the second dma_set_mask(). The only way to get to second
dma_set_mask() call is by an error code in the first one.

So I hereby remove the check since it looks superfluous. Please ignore
the path if there is black magic involved.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>