Bluetooth: controller: Fix CONFIG_BT_CTLR_TX_BUFFER_SIZE range
Bluetooth: controller: Fix CONFIG_BT_CTLR_TX_BUFFER_SIZE range

Fix CONFIG_BT_CTLR_TX_BUFFER_SIZE value range to 251 Bytes
due to implementation limitation in use of u8_t for PDU
length fields in controller Tx buffers.

Signed-off-by: Vinayak Kariappa Chettimada <e780c9f87dc729ad454f47292ad82c8652353064@nordicsemi.no>

zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr