drivers: serial: uart_stellaris: Remove UART_IRQ_FLAGS

We always have UART_IRQ_FLAGS set to 0, so just call IRQ_CONNECT with a
0 argument for the flags, and remove the UART_IRQ_FLAGS.  This is
towards support for using the driver on the TI CC2650.  (we add a
comment about that as well).

Signed-off-by: Geoffrey Le Gourriérec <geoffrey.legourrierec@smile.fr>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>