ubifs: auth.c: fix kernel-doc function prototype warning

Use the correct function name in the kernel-doc comment to prevent
a kernel-doc warning:

auth.c:30: warning: expecting prototype for ubifs_node_calc_hash(). Prototype was for __ubifs_node_calc_hash() instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: lore.kernel.org/r/202311052125.gE1Rylox-lkp@intel.com
Cc: Richard Weinberger <richard@nod.at>
Cc: linux-mtd@lists.infradead.org
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>