drivers: serial: nrfx_uarte2: Fix re-enabling the receiver

Move providing of the first buffer in uart_rx_enable from that
function to the callback which is called from nrfx_uarte_rx_enable
context. By doing this nrfx_uarte_rx_buffer_set is called when
RX configuration flags are already known to the driver.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>