gp8psk: Fix DVB frontend attach

gp8psk: Fix DVB frontend attach

The DVB binding schema at the DVB core assumes that the frontend is a
separate driver.  Faling to do that causes OOPS when the module is
removed, as it tries to do a symbol_put_addr on an internal symbol,
causing craches like:

    WARNING: CPU: 1 PID: 28102 at kernel/module.c:1108 module_put+0x57/0x70
    Modules linked in: dvb_usb_gp8psk(-) dvb_usb dvb_core nvidia_drm(PO) nvidia_modeset(PO) snd_hda_codec_hdmi snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_timer snd soundcore nvidia(PO) [last unloaded: rc_core]
    CPU: 1 PID: 28102 Comm: rmmod Tainted: P        WC O 4.8.4-build.1 #1
    Hardware name: MSI MS-7309/MS-7309, BIOS V1.12 02/23/2009
    Call Trace:
       dump_stack+0x44/0x64
       __warn+0xfa/0x120
       module_put+0x57/0x70
       module_put+0x57/0x70
       warn_slowpath_null+0x23/0x30
       module_put+0x57/0x70
       gp8psk_fe_set_frontend+0x460/0x460 [dvb_usb_gp8psk]
       symbol_put_addr+0x27/0x50
       dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]

From Derek's tests:
    "Attach bug is fixed, tuning works, module unloads without
     crashing. Everything seems ok!"

Reported-by: Derek <user.vdr@gmail.com>
Tested-by: Derek <user.vdr@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>