drivers: video: rename 8 bit bayer format in Sxxxx

In order to be consistant with other formats bayer formats
recently introduced, rename all 4 8 bit bayer formats with a
S prefix (SBGGR / SRGGB / SGBRG / SGRBG).

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>