sparc32 protect read-only bits in DMA CSR registers

On a real hardware changing read-only bits has no effect
Use a mask common for SCSI and Ethernet registers. The crucial
bit is DMA_INTR, because setting or clearing it may produce
spurious interrupts.

This patch allows booting Solaris 2.3

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
