ieee802154: ca8210: Add checks for kmalloc allocation failures

Ensure we don't end up with a null pointer dereferences by checking
for for allocation failures.  Allocate by sizeof(*ptr) rather than
the type to fix checkpack warnings.  Also merge multiple lines into
one line for the kmalloc call.

Detected by CoverityScan, CID#1422435 ("Dereference null return value")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>