fb_open returns a pointer and doesn't populate errno, which will
result in a stale errno being evaluated by perror() on failure.
Fix this by using strerror with the proper argument instead at call sites.

While at it, correct the message prefix typo (s/fb_open/fb_open/).

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>