bluetooth: tester: Bump supported GATT Prepare Writes to 12

This is required by GATT/SR/GAW/BV-14-C where PTS does a lot of small
writes to characteristic of 512 bytes.

Signed-off-by: Szymon Janc <[email protected]>

Author: sjanc

tests/bluetooth/tester/prj.conf

@@ -13,7 +13,7 @@ CONFIG_BT_SMP_ALLOW_UNAUTH_OVERWRITE=y
 CONFIG_BT_SMP_APP_PAIRING_ACCEPT=y
 CONFIG_BT_SIGNING=y
 CONFIG_BT_BONDABLE=y
-CONFIG_BT_ATT_PREPARE_COUNT=8
+CONFIG_BT_ATT_PREPARE_COUNT=12
 CONFIG_BT_GATT_CLIENT=y
 CONFIG_BT_L2CAP_DYNAMIC_CHANNEL=y
 CONFIG_BT_DEVICE_NAME="Tester"