Consolidate message buffer

[robin-nitrokey]

Currently, the message buffer in usbd-ctaphid is shorter than the message buffer used by ctaphid-dispatch. We should either use the same length for both buffers, or try to reuse the buffer used by ctaphid-dispatch in usbd-ctaphid.

See #2

[robin-nitrokey]

#20 makes ctaphid-dispatch and usbd-ctaphid use the same buffer size. We should still investigate if we can reuse the buffer instead of having two separate buffers.