staging: fbtft: Constify buf parameter in fbtft_dbg_hex()

It's obvious that we don't and shouldn't modify buffer that
is about to be dumped. Constify parameter in fbtft_dbg_hex()
to make it clear.

Fixes: c296d5f9 ("staging: fbtft: core support")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220316165351.58107-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>