NTB: Fix an error code in ntb_msit_probe()

When the value of nm->isr_ctx is false, the value of ret is 0.
So, we set ret to -ENOMEM to indicate this error.

Clean up smatch warning:
drivers/ntb/test/ntb_msi_test.c:373 ntb_msit_probe() warn: missing
error code 'ret'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>