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>