driver: uart stm32: Check irq enabled in API calls
driver: uart stm32: Check irq enabled in API calls

When calling irq_rx_ready or irq_tx_ready API, return the logical AND
between the irq status and the enable of that irq.

Signed-off-by: Nicolas VINCENT <bf1fab8e8ae5f19954e27cd4beab79cce48f8e9f@vossloh.com>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr