nvmem: u-boot-env: improve coding style

nvmem: u-boot-env: improve coding style

1. Prefer kzalloc() over kcalloc()
   See memory-allocation.rst which says: "to be on the safe side it's
   best to use routines that set memory to zero, like kzalloc()"
2. Drop dev_err() for u_boot_env_add_cells() fail
   It can fail only on -ENOMEM. We don't want to print error then.
3. Add extra "crc32_addr" variable
   It makes code reading header's crc32 easier to understand / review.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20231221173421.13737-5-zajec5@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>