rtl8180: fix DMA register are written two times

Hw DMA registers are written in rtl8180_init_hw function.
They are also written again just after calling rtl8180_init_hw.
There is no point in doing this twice.

Remove those redundant register writes from rtl8180_start.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>