Although `keychron/c2_pro/ansi/rgb` and `keychron/c2_pro/ansi/white` use
the same custom matrix code, the matrix layouts are slightly different;
in particular, only the `keychron/c2_pro/ansi/white` board actually uses
column 19. However, the handling of column 19 in the custom matrix code
was broken, therefore that column did not work.
Looks like the custom matrix code assumes that `SHIFT_COL_END` refers to
the last column connected to the shift register, and not to the column
past that; so the value of `SHIFT_COL_END` needs to be changed from 19
to 18 (columns 11...18 are connected to the shift register, and column
19 is connected to the C14 pin).
Also the code which was determining `SIZE_T` and `UNSELECT_ALL_COL` had
an off-by-one bug when counting the required number of bits (again due
to the confusion on the `SHIFT_COL_END` meaning); this had been fixed
too (the actual behavior of that part of the code did not change,
because both the old and the new version select the 8 bit variant).
