sh: Fix up the shared IRQ demuxer's control bit testing logic.

Correct the interrupt handler in sh4 serial device, return the correct
value and check for what is anabled in the SCSCR register. The sh7722 is
broken just sending a break using minicom.

Signed-off-by: Michael Trimarchi <trimarchimichael@yahoo.it>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>