ibmvnic: Fix unchecked return codes of memory allocations

The return values for these memory allocations are unchecked,
which may cause an oops if the driver does not handle them after
a failure. Fix by checking the function's return code.

Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>