Input: cyapa_gen6 - fix out-of-bounds stack access

gcc -Warray-bounds warns about a serious bug in
cyapa_pip_retrieve_data_structure:

drivers/input/mouse/cyapa_gen6.c: In function 'cyapa_pip_retrieve_data_structure.constprop':
include/linux/unaligned/access_ok.h:40:17: warning: array subscript -1 is outside array bounds of 'struct retrieve_data_struct_cmd[1]' [-Warray-bounds]
   40 |  *((__le16 *)p) = cpu_to_le16(val);
drivers/input/mouse/cyapa_gen6.c:569:13: note: while referencing 'cmd'
  569 |  } __packed cmd;
      |             ^~~

Apparently the '-2' was added to the pointer instead of the value,
writing garbage into the stack next to this variable.

Fixes: c2c06c41 ("Input: cyapa - add gen6 device module support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20201026161332.3708389-1-arnd@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/cyapa_gen6.c

@@ -573,7 +573,7 @@ static int cyapa_pip_retrieve_data_structure(struct cyapa *cyapa,
 
 	memset(&cmd, 0, sizeof(cmd));
 	put_unaligned_le16(PIP_OUTPUT_REPORT_ADDR, &cmd.head.addr);
-	put_unaligned_le16(sizeof(cmd), &cmd.head.length - 2);
+	put_unaligned_le16(sizeof(cmd) - 2, &cmd.head.length);
 	cmd.head.report_id = PIP_APP_CMD_REPORT_ID;
 	cmd.head.cmd_code = PIP_RETRIEVE_DATA_STRUCTURE;
 	put_unaligned_le16(read_offset, &cmd.read_offset);