drivers: serial: Fix uart_poll_in() for mcux flexcomm driver
drivers: serial: Fix uart_poll_in() for mcux flexcomm driver

Replaces kUSART_RxFifoFullFlag with kUSART_RxFifoNotEmptyFlag to
prevent Rx FIFO overrun.

Signed-off-by: Jiří Keresteš <1b548d10035ae428195c41f5d55eedb1437ccd8a@kerestes.cz>

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