drivers: bcm2835_unicam: Disable trigger mode operation

On a Pi3 B/B+ platform the imx219 sensor frequently generates a single corrupt
frame when the sensor first starts. This can either be a missing line, or
invalid samples within the line. This only occurrs using the Unicam kernel
driver.

Disabling trigger mode elimiates this corruption. Since trigger mode is a
legacy feature copied from the firmware driver and not expected to be needed,
remove it. Tested on the Raspberry Pi cameras and shows no ill effects.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>