fimc-is: Remove BUG() from fimc_is_itf_s_param()

fimc-is: Remove BUG() from fimc_is_itf_s_param()

The condition being tested for that BUG() invocation is often
recoverable, so replace BUG() with a WARN_ON() and return EINVAL.
This prevents specific user space call sequences crashing the kernel,
which should never happen.
The callers of fimc_is_itf_s_param() function will be updated in
subsequent patches if necessary, to prevent any further possible
crashes.

Change-Id: I90a08278baa0cee85a4a69fa1b278e28e5c6764a
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>