serial: stm32: improve platform_get_irq condition handling in init_port

serial: stm32: improve platform_get_irq condition handling in init_port

Replace "ret" variable by "irq" variable from platform_get_irq condition
handling in stm32_init_port as suggested by Jiri in "STM32 uart cleanup and
improvement" series review.
This change will prevent port->irq to be unexpectly modified by a potential
change of "ret" value introduced by a new patch.

Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210121142309.6327-1-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>