drivers: uart_nrfx_uart: Request next buffer only when needed

Recent refactoring of the uart_async_api test (see commit
eb44414a) revealed an issue
in the uart_nrfx_uart driver that it requested the next RX
buffer even if one was already set up (such request was just
ignored in the previous form of the test, so the problem did
not come out so far).
This patch prevents such incorrect requests from appearing.

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