Annex_M has to be configured at the DSLAM.
I'm going to say there are no settings to turn it off... because strictly speaking there isnt anything in any of the modem GUIs.
However,
If you did have Annex_M, there is a dirty hack that may work in the CLI for some modems, see
Speedtouch 585 AnnexA / Annex M. Although I used DMT tool to make it easier, there will be direct CLI commands.
1.2 Mbps isnt indication of Annex M. It depends on the DSLAM manufacturer and how the SP has configured the useable bitstream. As Meeritez mentioned its just the top end of ADSL2+, if you did have annex_m then you'd be looking at about a min of 2Mb when paired with a 17Mbos down. (Due to spectral shaping, it's not a straight swap)
A loss of downstream sync from 17 to 15 Mbps is probably about right for interleaving overheads. In fact depending upon the interleave depth it could be more.
I#m fairly confident to say that you do not have annex_m