[media] radio-bcm2048: don't ignore errors

Remove this warning:

drivers/staging/media/bcm2048/radio-bcm2048.c: In function 'bcm2048_set_rds_no_lock':
drivers/staging/media/bcm2048/radio-bcm2048.c:467:6: warning: variable 'err' set but not used [-Wunused-but-set-variable]
  int err;
      ^~~

By returning the error code.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>