Bluetooth: Validate data size before accessing mgmt commands

Crafted (too small) data buffer could result in reading data outside of buffer.
Validate buffer size and return EINVAL if size is wrong.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Acked-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>