auxdisplay: hd44780: Rename hd to hdc in hd44780_common_alloc()

The hd44780_common_alloc() uses hd for local variable while
the respective header uses hdc, rename to make it consistent
and avoid potential confuse with the drivers that use both
for different reasons. No functional changes intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>