kernel/init.c: missing memset() to early_memset() conversion

Commit 678b76e4b032 ("kernel/init.c: allow for memset/memcpy
alternatives during early boot") and commit da28829b6451 ("kernel:
zero the bss section of OCM memory at boot time") were created
independently and missed changes from each other.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>