auxdisplay: img-ascii-lcd: Convert device attribute to sysfs_emit()

Convert the "message" device attribute from sprintf() to sysfs_emit(),
as the latter is aware of the PAGE_SIZE buffer.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>