staging: et131x: Fix 64bit tx dma address handling

The driver checks that the device can handle 64bit DMA addressing in
et131x_pci_setup(), but then assumes that the top dword of a tx dma
address is always zero when creating a dma mapping in nic_send_packet().
Fix the mapping to use the higher dword of the dma_addr_t returned by
dma_map_single() and skb_frag_dma_map().

Also remove incorrect comments stating that dma_map_single() only returns
a 32 bit address.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>