serial: core: remove redundant assignment to port

The final assignment to port is never read, hence it is redundant
and can be removed. Also move the declaration of port to a more
local scope.  Cleans up clang warning:

drivers/tty/serial/serial_core.c:1498:2: warning: Value stored
to 'port' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>