drm/ssd130x: Fix pre-charge period setting

Fix small typo which causes the mask for the 'precharge1' setting
to be used with the 'precharge2' value.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220706184133.210888-1-ezequiel@vanguardiasur.com.ar