staging/atomisp: fix empty-body warning

Defining a debug function to nothing causes a warning with an empty block
after if()/else():

drivers/staging/media/atomisp/i2c/ov2680.c: In function 'ov2680_s_stream':
drivers/staging/media/atomisp/i2c/ov2680.c:1208:55: error: suggest braces around empty body in an 'else' statement [-Werror=empty-body]

This changes the empty debug statement to dev_dbg(), which by default also
does nothing, but avoids this warning and also checks the format string.
As a side-effect, we can now use dynamic debugging to turn on the
output at runtime.

Fixes: a49d2536 ("staging/atomisp: Add support for the Intel IPU v2")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>