media: i2c: ov5647: Add control of V4L2_CID_HBLANK

The driver did expose V4L2_CID_HBLANK, but as a READ_ONLY control.

The sensor only uses the HTS register to control the line length,
so convert this control to read/write, with the appropriate ranges.
Adopt the old fixed values as the minimum values permitted in each
mode to avoid issues of it not streaming.

This should allow exposure times up to ~3 seconds (up from ~1sec).

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