V4L/DVB (6495): saa7146: saa7146_wait_for_debi_done fixes

Two fixes for the 'saa7146_wait_for_debi_done' code:
(a) Timeout did not work when the routine was called with
    interrupts disabled.
(b) Reduce PCI I/O load caused by saa7146_wait_for_debi_done.
    Seems to be very important on fast machines!

Based on code posted by Hartmut Birr @vdr-portal.


Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>