net: ngbe: fix NULL pointer dereference in non-MSI-X interrupt enabling

net: ngbe: fix NULL pointer dereference in non-MSI-X interrupt enabling

In non-MSI-X mode (such as legacy INTx or single MSI), wx->msix_entry is
not allocated or initialized. Calling NGBE_INTR_MISC(wx) dereferences
wx->msix_entry->entry, leading to a NULL pointer dereference crash.

This issue was introduced by fixing the IRQ vector when the number of
VFs is 7. Fix the issue by explicitly checking `pdev->msix_enabled` to
determine the correct vector index.

Additionally, as a side fix, set the interrupt mask to BIT(0) for the
non-MSI-X fallback. In MSI/INTx mode, the MISC and queue interrupts
share vector 0, and the WX_PX_MISC_IVAR register is only valid in the
MSI-X case. Thus, BIT(0) is the correct mask for the miscellaneous cause
when MSI-X is disabled.

Fixes: 4174c0c331a2 ("net: ngbe: specify IRQ vector when the number of VFs is 7")
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/B2693E9A8BFAD110+20260807062214.410838-1-jiawenwu@trustnetic.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>