Blackfin Serial Driver: Fix bug - kernel hangs when accessing uart 0 on bf537...

Blackfin Serial Driver: Fix bug - kernel hangs when accessing uart 0 on bf537 when booting u-boot and linux on uart 1

Wait only when this UART is enabled.
http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3995



Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>