V4L/DVB (5020): Fix: disable interrupts while at KM_BOUNCE_READ

vivi.c uses the KM_BOUNCE_READ with local interrupts enabled. 
This means that if a disk interrupt occurs while vivi.c is using this
fixmap slot, the vivi.c driver will, upon return from that interrupt, find
that the fixmap slot now points at a different physical page.
The net result will probably be rare corruption of disk file contents,
because viv.c will now be altering the page which the disk code was
recently using. 

Thanks to Andrew Morton for pointing this.
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>