modem: backends: uart_isr: improve the reception of bytes

Add a configurable delay between when a byte is received and
MODEM_PIPE_EVENT_RECEIVE_READY is sent.

This fixes data reception at baud rates above 460800, and
most likely also reduces the workload at any baud rate
when receiving bytes by not going through the work item
and callbacks for every single byte.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>