drivers: uart_nrfx_uart: Replace k_delayed_work with k_timer

There is no need to involve a work queue to control timeouts in
the uart_nrfx_uart driver. Kernel timers are sufficient for those
tasks, and the uart_nrfx_uarte driver may serve as a proof of this.
Replace then those uses of work queue with timers, for consistency
and simplicity, also to avoid potential issues with cancellation of
the delayed work queue items.

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