keymap: Fix Samsung 900X[34]C

It appears that it's not really the 900 vs. 940 or the X3 vs X4, but the
A/B/C/D suffix after that which makes the difference between the keymaps. On a
NP900X3C-A04RU you get

   MODALIAS=dmi:bvn*:bvr*:bd*:svnSAMSUNGELECTRONICSCO.,LTD.:pn900X3C/900X3D/900X4C/900X4D:*

So change the matches to use AB vs. CDG as the differentiator.

Thanks to Anatoly Markov for reporting and testing!