fbdev: clean up exit patch of fb_set_var

Clean up exit patch of fb_set_var():

- consolidate all return values into a single local variable
- ensure that return values are valid error codes
- fix fb_set_var() returning success when fb_check_caps() failed

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>