pci_ep: Fix Coverity warning

Fix the following Coverity warning:
CID 244086:  Incorrect expression  (BAD_COMPARE)
Comparing pointer "ep_bar" against NULL using anything besides == or
is likely to be incorrect.

Fixes: 914026d2 ("drivers: pci_ep: Introduce UCLASS_PCI_EP uclass")

Signed-off-by: Ramon Fried <rfried.dev@gmail.com>