auxdisplay: lcd2s: Fix lcd2s_redefine_char() feature

It seems that the lcd2s_redefine_char() has never been properly
tested. The buffer is filled by DEF_CUSTOM_CHAR command followed
by the character number (from 0 to 7), but immediately after that
these bytes are rewritten by the decoded hex stream.

Fix the index to fill the buffer after the command and number.

Fixes: 8c9108d0

 ("auxdisplay: add a driver for lcd2s character display")
Cc: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
[fixed typo in commit message]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>