Update the documented value range that `uart_irq_tx_ready` returns in
order to provide more information to the callers about the number of
bytes that could be writted with `uart_fifo_fill` without fragmentation.

Signed-off-by: Jordan Yates <jordan@embeint.com>

Update the interrupt driver UART drivers that use `struct ring_buf`
internally to report the number of bytes that can be pushed in
`uart_fifo_fill` without fragmentation.

Signed-off-by: Jordan Yates <jordan@embeint.com>

The async IRQ shim supports writes of any size up to the TX buffer size.

Signed-off-by: Jordan Yates <jordan@embeint.com>

Update nrfx drivers with the minimum number of bytes that can be sent
in a single call to `uart_fifo_fill`.

Signed-off-by: Jordan Yates <jordan@embeint.com>