drivers: net: ppp: improve CONFIG_NET_PPP_ASYNC_UART_TX_TIMEOUT

Its default value (100 ms) resulted in PDUs big enough to never make it
through on a slow enough UART (e.g. ~1152-byte PDUs on a UART@115200).
The UART TXs were silently aborted.

A no-timeout value is now allowed and made the default.
Additional warnings are logged when it is likely that a UART TX
was aborted due to a too low timeout for the used baud rate.

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