auxdisplay: lcd2s: Allocate memory for custom data in charlcd_alloc()

Allocate memory for custom data in charlcd_alloc() instead of doing that
explicitly in the driver.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>