V4L/DVB (5244): Dvbdev: fix illegal re-usage of fileoperations struct

Arjan van de Ven <arjan@infradead.org> reported an illegal re-usage of 
the fileoperations struct if more than one dvb device (e.g. frontend) is 
present. 

This patch fixes this issue.  

It allocates a new fileoperations struct each time a device is 
registered and copies the default template fileops.

Signed-off-by: Marcel Siegert <mws@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>