Fix bug in io64 target (introduced by commit aba27acf)

From comparison of current logic and the logic that was prior to commit
aba27acf, we see that first parameter of FPGA_GET_REG() shall be the
FPGA index and not channel number. The re-factoring in commit aba27acf
accidentally changed that.

Cc: Stefan Roese <sr@denx.de>
Acked-by: Dirk Eibach <dirk.eibach@gdsys.cc>
Signed-off-by: Vasili Galka <vvv444@gmail.com>