staging: dgrp: Drop unnecessary typecast

An unnecessary typecast in dgrp_net_ops.c causes the following build error
if compiled with W=1.

In function ‘copy_from_user’, inlined from ‘dgrp_net_ioctl’ at
    drivers/staging/dgrp/dgrp_net_ops.c:3408:21:
    arch/x86/include/asm/uaccess_32.h:211:26: error: call to
    ‘copy_from_user_overflow’ declared with attribute error: copy_from_user()
    buffer size is not provably correct

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>