staging: dgnc: Fix externs should be avoided in the .c files

This commit fixes the following checkpatch warnings:

WARNING: externs should be avoided in .c files
    #80: FILE: drivers/staging/dgnc/dgnc_driver.c:80:
        +int            dgnc_init_module(void);
    #81: FILE: drivers/staging/dgnc/dgnc_driver.c:81:
        +void           dgnc_cleanup_module(void);

This was caused by putting the declarations for module init and module
exit fucntions on the top of the file. The fix removes these
declarations plus it also corrects the type of the init/exit functions.

Due to the dependency between init and exit functions the
dgnc_cleanup_module had to be put first.

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>