net: mvpp2: simplify IRQ validity check

irq_of_parse_and_map() returns 0 when parsing or mapping an IRQ fails.
The current condition also checks for negative values, even though
irq_of_parse_and_map() does not return negative error codes.

Check only for a zero return value to match the semantics of
irq_of_parse_and_map().

Signed-off-by: bui duc phuc <phucduc.bui@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260903075753.346647-1-phucduc.bui@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>