drivers: wifi: esp_at: fix AT+CIPSEND premature timeout

For TCP socket, this fixes AT+CIPSEND command with too short timeout.
ESP modem replies SEND OK/SEND FAIL dependent on network traffic
condition, so this timeout config changes as Kconfig option for being
configurable by user.

Signed-off-by: Chun-Chieh Li <ccli8@nuvoton.com>