mtd: rawnand: use kcalloc() instead of kzalloc()

mtd: rawnand: use kcalloc() instead of kzalloc()

We are trying to get rid of all multiplications from allocation
functions to prevent integer overflows[1]. Here the multiplication is
obviously safe, but using kcalloc() is more appropriate and improves
readability. This patch has no effect on runtime behavior.

Link: https://github.com/KSPP/linux/issues/162
Link: https://www.kernel.org/doc/html/next/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>