octeontx2-af: cn10k: mcs: Fix error return code in mcs_register_interrupts()

If alloc_mem() fails in mcs_register_interrupts(), it should return error
code.

Fixes: 6c635f78 ("octeontx2-af: cn10k: mcs: Handle MCS block interrupts")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>