Bluetooth: Host: Disallow sending unsupported MTU Exchange
Bluetooth: Host: Disallow sending unsupported MTU Exchange

The Exchange MTU sub-procedure shall only be supported on the
LE Fixed Channel Unenhanced ATT bearer.

This was checked when receiving, but sending was allowed.

Signed-off-by: Herman Berget <22d956f84c179c5c22821e4925fbb7a703726077@nordicsemi.no>

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