Keychron Q11 uses `matrix_mask` to mask away the matrix location used
for `SPLIT_HAND_MATRIX_GRID` (otherwise the keyboard will constantly
generate a wakeup request whenever the host enters suspend). However,
the value of `matrix_mask` is actually chosen to mask away all unused
locations, but it was apparently generated for the ANSI board and
therefore masks away the left Shift key on the ISO board, breaking the
functionality of that key.
Fix `matrix_mask` to make the masking work as intended:
- Set the `[4, 1]` bit to 1; this is the matrix location for the ISO
left Shift.
- Set the `[2, 5]` bit to 0; this is actually the matrix location which
corresponds to `#define SPLIT_HAND_MATRIX_GRID A2, A15` on the left
side, which was intended to be masked; apparently the wakeup problem
does not happen due to `SPLIT_HAND_MATRIX_GRID_LOW_IS_RIGHT`, so the
left half actually has that matrix location open. On the right side
`A2, A15` is `[9, 6]`, which was masked off correctly.
