serial: lpc32xx: Fix fallout from tty_port conversion

A duplicate definition of the port variable was introduced in the
interrupt handler, which causes the build to break. The fix is to
rename the variable to tport, which is already properly used in
subsequent code.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>