[media] v4l: rcar_fdp1: use %4.4s to format a 4-byte string

Using %4s to format f->fmt.pix_mp.pixelformat in fdp1_try_fmt() and
fdp1_s_fmt() may lead to more characters being printed (when the byte
following field pixelformat is not zero).

Add ".4" to the format specifier to limit the number of printed
characters to four. The resulting format specifier "%4.4s" is also used
by other media drivers to print pixelformat value.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Reviewed-by: Kieran Bingham <kieran@bingham.xyz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>