Bluetooth: tests: edtt: fix buffer overflow error

HCI/GEV/BV-01-C tries to send 255 bytes over HCI, as part of sending
an unknown/unsupported command, but the default buffer size
is 65, which results in a buffer overflow and undefined behaviour.
Instead of crashing hard we now check the buffer length.
In order for EDTT tests to pass we set the buffer size to 255

Signed-off-by: Andries Kruithof <Andries.Kruithof@nordicsemi.no>