e100: Add a check after pci_pool_create to avoid null pointer dereference

The driver lacks the check of nic->cbs_pool after pci_pool_create
in e100_probe. When this function is failed, a null pointer dereference
occurs when pci_pool_alloc uses nic->cbs_pool in e100_alloc_cbs.
This patch adds a check and related error handling code to fix it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>