crypto: stm32/crc32 - fix parsing list of devices

smatch warnings:
drivers/crypto/stm32/stm32-crc32.c:108 stm32_crc_get_next_crc() warn:
can 'crc' even be NULL?

Use list_first_entry_or_null instead of list_first_entry to retrieve
the first device registered.
The function list_first_entry always return a non NULL pointer even if
the list is empty. Hence checking if the pointer returned is NULL does
not tell if the list is empty or not.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202311281111.ou2oUL2i-lkp@intel.com/


Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202311281111.ou2oUL2i-lkp@intel.com/


Signed-off-by: Thomas Bourgoin <thomas.bourgoin@foss.st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>