video: ssd1307fb: Introduce temporary variable to increase readability

Introduce temporary variable to increase readability of the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[b.zolnierkie: fix lines over 80 characters]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200324170532.44384-2-andriy.shevchenko@linux.intel.com