drivers: wifi: esp: support 32-bit length in +IPD

+IPD,<sock>,<bytes_avail> command in passive mode notifies about ESP
internal buffer usage level. Hence value of <bytes_avail> can exceed
4 digit value, which currently results in failure.

+IPD message is used in driver only for scheduling AT+CIPRECVDATA
command and actual value of <bytes_avail> doesn't really
matter. Increase maximum supported value that can be received from
9999 (maximum 4 digits) to 4294967295, which is maximum value of 32-bit
unsigned integer.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>