rtl8180: change PCI DMA mask to DMA_BIT_MASK(32)

From the original report:

"I had problems to get my rtl8185 PCI card running on Sparc64: I always
got an error about "No suitable DMA available" followed by an error
that no device could be detected. When comparing the rtl8180 driver to
others I noticed that others are mostly using DMA_BIT_MASK so I changed
the custom mask to DMA_BIT_MASK(32) which fixed my issue."

Reported-by: Tiziano Müller <tm@dev-zero.ch>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>