fbdev: ssd1307fb: fix logical error

The logical not needs to be done after the bit masking.

Fixes: a3998fe03e87 ("fbdev: ssd1307fb: Unify init code and obtain
hw specific bits from DT")

Signed-off-by: Thomas Niederprüm <niederp@physik.uni-kl.de>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>