staging: vt6655: remove unused macro

The MAC_MAX_CONTEXT_SIZE macro was not enclosed into parenthesis,
which might have caused hard to debug errors, and caused a sparse
warning.

Since it is unused, we might as well remove it.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>