caif: fix a potential NULL dereference

Commit bd30ce4bc0b7 (caif: Use RCU instead of spin-lock in caif_dev.c)
added a potential NULL dereference in case alloc_percpu() fails.

caif_device_alloc() can also use GFP_KERNEL instead of GFP_ATOMIC.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Sjur Brændeland <sjur.brandeland@stericsson.com>
Acked-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>