omap2/3: make serial_in_override() address the right uart port

Commit f62349ee

 makes it possible to
have some other than first uart port as ttyS0, which breaks the workaround
serial_in_override() function which will try to address the first uart
port (for ttyS0) and not the one that was initialized.

Signed-off-by: Alexander Shishkin <virtuoso@slind.org>
CC: Mika Westerberg <ext-mika.1.westerberg@nokia.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>