drm/ssd130x: fix ssd132x_clear_screen() columns

The number of columns relates to the width, not the height.  Use the
correct variable.

Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Fixes: fdd591e0 ("drm/ssd130x: Add support for the SSD132x OLED controller family")
Link: https://lore.kernel.org/r/20250611111307.1814876-1-jkeeping@inmusicbrands.com


Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>