skeletonfb: more corrections

More corrections to skeletonfb.c file:
- fixed types of more arguments
- removed returned values in void functions
- removed not existing fb_poll function
- fixed closing comment typo
- corrected misleading constant name ADDR

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Acked-By: James Simmons <jsimmons@infradead.org>
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>