drivers: serial: gecko: Avoid getting masked interrupts

The USART_IntGet() function seems to return masked interrupts in the
function uart_gecko_irq_tx_ready() where it should return only
non-masked interrupts. Using USART_IntGetEnabled() fixes this issue.

This issue was discovered when using the mcumgr UART backend in the
uart_mcumgr_isr() function.

Signed-off-by: Steven Lemaire <steven.lemaire@zii.aero>