V4L/DVB (9513): cx18: Reduce number of mmio read retries

cx18: Reduce number of mmio read retries to improve performance.  Experiments
have shown 2 things: read retries never improve the result of a suspect mmio
read from the CX23418 (the result stays all 0xff's), and that most of the
suspected read failures are actually proper reads of values that should be
all 0xff's.  This change reduces the number of read retries and keeps the
count separate from write retries.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>