media: bcm2835-unicam: Replace hard coded loop limit with a define

In order to protect against a dodgy sensor driver never returning
an error from enum_mbus_code there was a hardcoded value of 128
that aborted the loop.
There is a need to call enum_mbus_code from elsewhere, so move that
number to a define so it is common across calls.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>