i3c: master: Fix build error

Fix build error caused by commit 2aac0bf4 ("i3c: Add fallback method
for GETMXDS CCC") which incorrectly access the "struct i3c_ccc_cmd_dest
dest" as pointer.

drivers/i3c/master.c: In function ‘i3c_master_getmxds_locked’:
drivers/i3c/master.c:1140:21: error: invalid type argument of ‘->’ (have ‘struct i3c_ccc_cmd_dest’)
 1140 |                 dest->payload.len -= 3;
      |                     ^~

Fixes: 2aac0bf4

 ("i3c: Add fallback method for GETMXDS CCC")
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20231117110924.634280-1-jarkko.nikula@linux.intel.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>