serial: uart_stellaris: remove export of uart_stellaris_isr

There isn't any reason to export the uart_stellaris_isr function, so
lets make it static and remove the associated header file.

Change-Id: I3a131b584d9d6fb6279a1503512668a71510dd4d
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>