NTB: ntb_hw_idt: replace IS_ERR_OR_NULL with regular NULL checks

Both devm_kcalloc() and devm_kzalloc() return NULL on error. They
never return error pointers.

The use of IS_ERR_OR_NULL is currently applied to the wrong
context.

Fix this by replacing IS_ERR_OR_NULL with regular NULL checks.

Fixes: bf2a952d ("NTB: Add IDT 89HPESxNTx PCIe-switches support")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>