drivers: serial: Update cc32xx uart interrupt handling

The Zephyr console and shell interrupt processing assumes
a TX interrupt is evoked upon first enabling the TX
interrupt via uart_irq_tx_enable.

This was not the case with the cc32xx uart, coming out of
reset, with FIFO's disabled.

The only way found to achieve this behavior is to fill
the fifo with a non-printable character on initialization.

Also, the uart driver was explicitly clearing TX/RX interrupts in
its isr, which was unnecessary, as the act of reading/writing
did that implicitly.

These fixes allow the cc32xx uart to work with the
current Zephyr console/shell design.

Fixes: #11202

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>