Bluetooth: Remove unnecessary check

The function already fails if the given size is greater than the MTU, so
there is no need to consider that case afterwards.

Signed-off-by: Mikel Astiz <mikel.astiz.oss@gmail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>