[media] fimc-is-param: get rid of warnings

In file included from drivers/media/platform/exynos4-is/fimc-is-param.c:31:0:
drivers/media/platform/exynos4-is/fimc-is-errno.h:245:20: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
 const char * const fimc_is_strerr(unsigned int error);
                    ^
drivers/media/platform/exynos4-is/fimc-is-errno.h:246:20: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
 const char * const fimc_is_param_strerr(unsigned int error);
                    ^
drivers/media/platform/exynos4-is/fimc-is-param.c: In function 'fimc_is_set_initial_params':
drivers/media/platform/exynos4-is/fimc-is-param.c:670:23: warning: variable 'sensor' set but not used [-Wunused-but-set-variable]
  struct sensor_param *sensor;
                       ^

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>