auxdisplay: img-ascii-lcd: Fix doc comment to silence warnings

Compiling with W=1 with gcc 7.2.0 gives 2 warnings:

  drivers/auxdisplay/img-ascii-lcd.c:233: warning: Function parameter or
  member 't' not described in 'img_ascii_lcd_scroll'
  drivers/auxdisplay/img-ascii-lcd.c:233: warning: Excess function
  parameter 'arg' description in 'img_ascii_lcd_scroll'

Cc: Paul Burton <paul.burton@mips.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

drivers/auxdisplay/img-ascii-lcd.c

@@ -224,7 +224,7 @@ MODULE_DEVICE_TABLE(of, img_ascii_lcd_matches);
 
 /**
  * img_ascii_lcd_scroll() - scroll the display by a character
- * @arg: really a pointer to the private data structure
+ * @t: really a pointer to the private data structure
  *
  * Scroll the current message along the LCD by one character, rearming the
  * timer if required.