STM32F0: manage better uart3 & 4 irqs
STM32F0: manage better uart3 & 4 irqs

Manage the case where both uart3 and uart4 interrupts are arriving at the same time.

c1728p9/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,mbedmicro/mbed,betzw/mbed-os,andcor02/mbed-os,mbedmicro/mbed,betzw/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,betzw/mbed-os,karsev/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,karsev/mbed-os,karsev/mbed-os,karsev/mbed-os,betzw/mbed-os,andcor02/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,betzw/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,karsev/mbed-os,karsev/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os