drivers: serial: stm32: report only unmasked irq

Fix #5298
irq_is_pending function returned TXE/RXNE flag status
even if IRQ was masked, which led to enless loop
in uart_pipe when no TX was performed. Fix by reporting status only when
IRQ is unmasked.

Signed-Off-by: Erwan Gouriou <erwan.gouriou@linaro.org>