Bluetooth: Kconfig: Set BT_BUF_EVT_RX_SIZE to 255 for BR/EDR

The maximum data length of event packet is 255 for BR/EDR.
Such as the data length of event Remote Name Request
Complete is 255.

Set BT_BUF_EVT_RX_SIZE to 255 to avoid error "Not enough
space in buffer" from bt_hci driver.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>