shell: backend: Set RX size default to 256 if shell MCUmgr is enabled

This fixes an issue whereby when USB CDC is used for receiving MCUmgr
commands, the commands are corrupted, invalid or messed up by
increasing the receive buffer size so that it can handle at least 1
full MCUmgr fragment.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>