[SERIAL] sunzilog: Interrupt enable before ISR handler installed

This patch changes the interrupt enable sequence for the sunzilog driver 
so that interrupts are not enabled untill after the interrupt handler has 
been installed. If this is not done, some SS1 and SS2 sun4c systems panic 
on un-handled interrupt before the handler gets installed preventing boot.

It also adds in support for the ESCC version of the zilog chips. The
changes mean that the FIFO will be enabled for ESCC versions of the
SCC UART. My interpretation of the SCC manual and the existing interrupt
handler code is that it sould be able to make good use of the FIFO without
issues.

Signed-off-by: Mark Fortescue <mark@mtfhpc.demon.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>