synclink_gt fix missed serial input signal changes

Fix missed serial input signal changes caused by rereading the serial
status register during interrupt processing.  Now processing is performed
on original status register value.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>