[SERIAL] 8250 woraround for buggy uart

[SERIAL] 8250 woraround for buggy uart

Patch from Alex Williamson

This patch adds support for detecting and working around a bug in
the A2 rev of the Exar ST16C2550 UART.  The chip incorrectly
advertises an EFR and mis-detects as having the wrong size FIFO.
Much of the patch below is Russell's proposed solution to the
problem.  The only changes I've made are to check the FIFO size
on the part (because there is a real part with the same divisor
ID and larger FIFO) and save and restore the LCR register around
the size_fifo() routine (it doesn't work correctly with a LCR
value of 0xBF).

Signed-off-by: Alex Williamson
Signed-off-by: Russell King