drivers: uart_nrfx_uarte: Fix call to HW_RX_COUNTING_ENABLED()

Since the macro references `data->async->hw_rx_counting`, it cannot
be called when `data->async` is NULL, and this could happen when the
PM_DEVICE_ACTION_RESUME action was requested for an instance that uses
the interrupt-driven API while for another instance that uses the
asynchronous API the hardware counting of bytes was configured.
Prevent this by calling the macro from a proper `if` block.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>