fbdev/ssd1307fb: fix optional VBAT support

SSD1306 needs VBAT when it is wired in charge pump configuration only.
Other controllers of the SSD1307 family do not need it at all. This was
introduced by commit ba14301e

 ("fbdev/ssd1307fb: add support to
enable VBAT").

Without VBAT configuration the driver now fails with:

	failed to get VBAT regulator: -19

This is caused by misinterpretation of devm_regulator_get_optional
which "returns a struct regulator corresponding to the regulator
producer or IS_ERR() condition".

Handle -ENODEV without bailing out and making VBAT support really
optional.

Signed-off-by: Bastian Stender <bst@pengutronix.de>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Jyri Sarha <jsarha@ti.com>
Cc: Roger Quadros <rogerq@ti.com>
[b.zolnierkie: minor fixups]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>