wpa_cli: Increase event buffer size to 4096 bytes

wpa_cli: Increase event buffer size to 4096 bytes

Number of other buffers were already increased to this size, but the
buffer used for receiving unsolicited event messages from wpa_supplicant
(e.g., for wpa_cli action scripts) was still at the older 256 byte size.
This could result in some events getting truncated. Avoid this by using
the same 4096 byte buffer size here as in the other places receiving
messages from wpa_supplicant.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>