media: i2c: imx477: Correct minimum exposure lines

The minimum number of exposure lines value (IMX477_EXPOSURE_MIN) was
previously 20 but this is not correct. The datasheet is not completely
explicit, however the new value of 4 has been tested with all the
sensor modes supported by this driver, and matches the lowest exposure
value of 114us that could be achieved wtih Raspberry Pi's legacy
firmware driver.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>