drivers: i3c: shell: fix compilation error in `cmd_i3c_ccc_getmxds`

Resolved a compilation error where `maxwr`, `maxrd`, and
`max_read_turnaround` were incorrectly referenced.

The code was updated to correctly access these fields from
the `data_speed` structure.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>