doc: fix uinput device creation warnings

doc: fix uinput device creation warnings

The example code for creating uinput devices produces the following
warnings:

warning: unused variable ‘ev’ [-Wunused-variable]
   | struct input_event ev[2];
   |                    ^~
warning: unused variable ‘new_fd’ [-Wunused-variable]
   | int fd, new_fd, uifd;
   |         ^~~~~~
warning: ‘err’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   |     return err;
   |            ^~~

Signed-off-by: José Expósito <jose.exposito89@gmail.com>