sparc escc IUS improvements (SunOS 4.1.4 fix)

According to scc_escc_um.pdf:
 - Reset Highest IUS must update irq status to allow processing
   of the next priority interrupt.
 - rx interrupt has always higher priority than tx on same channel

The documentation only explicitly says that Reset Highest IUS
command (0x38) clears IUS bits, not that it clears the corresponding
interrupt too, so don't clear interrupts on this command.

The patch allows SunOS 4.1.4 to use the serial ports

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
(cherry picked from commit 9fc391f8)