auxdisplay: hd44780: Call charlcd_alloc() from hd44780_common_alloc()

HD44780 APIs all operate on struct charlcd objects. Moreover, the current users
always call charlcd_alloc() and hd44780_common_alloc(). Make the latter call
the former, so eliminate the additional allocation, to make it consistent with
the rest of API and avoid duplication.

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