staging: greybus: camera: Use kcalloc for array's memory allocation.

Fix checkpatch warning:

WARNING: Prefer kcalloc over kzalloc with multiply

kcalloc is designed to allocate memory for arrays, its use is
preferable than kzalloc in these cases.

Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>