Add support for explicitly recognising QAM-256 with DVB-T2. This _may_ help with scanning on devices that don't support 'auto' modulation detection.

(cherry picked from commit 864d985786a6f0f0fe2b93b4539c12ee2b0e09e2)
(cherry picked from commit 376955a091dbf69f7cc233fa1bf3255401e80367)