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: c296d5f ("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>

Author: andy-shev

drivers/staging/fbtft/fbtft-core.c

@@ -49,7 +49,7 @@ int fbtft_write_buf_dc(struct fbtft_par *par, void *buf, size_t len, int dc)
 EXPORT_SYMBOL(fbtft_write_buf_dc);
 
 void fbtft_dbg_hex(const struct device *dev, int groupsize,
-		   void *buf, size_t len, const char *fmt, ...)
+		   const void *buf, size_t len, const char *fmt, ...)
 {
 	va_list args;
 	static char textbuf[512];

drivers/staging/fbtft/fbtft.h

@@ -240,7 +240,7 @@ struct fbtft_par {
 int fbtft_write_buf_dc(struct fbtft_par *par, void *buf, size_t len, int dc);
 __printf(5, 6)
 void fbtft_dbg_hex(const struct device *dev, int groupsize,
-		   void *buf, size_t len, const char *fmt, ...);
+		   const void *buf, size_t len, const char *fmt, ...);
 struct fb_info *fbtft_framebuffer_alloc(struct fbtft_display *display,
 					struct device *dev,
 					struct fbtft_platform_data *pdata);