samples: hci_uart: fix UART IRQ API usage

uart_irq_update() should always be called in IRQ processing callback.
From API documentation:
"This function should be called the first thing in the ISR. Calling
uart_irq_rx_ready(), uart_irq_tx_ready(), uart_irq_tx_complete()
allowed only after this."

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>