media: vivid: avoid integer overflow

Fixes these compiler warnings:

drivers/media/test-drivers/vivid/vivid-rds-gen.c: In function 'vivid_rds_gen_fill':
drivers/media/test-drivers/vivid/vivid-rds-gen.c:147:56: warning: '.' directive output may be truncated writing 1 byte into a region of size between 0 and 3 [-Wformat-truncation=]
  147 |         snprintf(rds->psname, sizeof(rds->psname), "%6d.%1d",
      |                                                        ^
drivers/media/test-drivers/vivid/vivid-rds-gen.c:147:52: note: directive argument in the range [0, 9]
  147 |         snprintf(rds->psname, sizeof(rds->psname), "%6d.%1d",
      |                                                    ^~~~~~~~~
drivers/media/test-drivers/vivid/vivid-rds-gen.c:147:9: note: 'snprintf' output between 9 and 12 bytes into a destination of size 9
  147 |         snprintf(rds->psname, sizeof(rds->psname), "%6d.%1d",
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  148 |                  freq / 16, ((freq & 0xf) * 10) / 16);
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Arnd Bergmann <arnd@arndb.de>