mtd: rawnand: Use kzalloc() instead of kmalloc() and memset()

Replace kmalloc() by a memset() followed with a kzalloc().

There is a recommendation to use zeroing allocator
rather than allocator followed by memset(0) in
./scripts/coccinelle/api/alloc/zalloc-simple.cocci

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>