lib/genalloc.c: correct dev_get_gen_pool documentation

The documentation mentions a "name" parameter, which does not exist.  This
commit removes such mention from the function documentation.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>