Partly revert "sfc: Handle serious errors in exactly one interrupt handler"

This reverts commit 63695459 in
drivers/net/ethernet/sfc/falcon.c.

Unlike the INT_ISR0 register on later controller revisions, the
NET_IVEC_INT_Q bits written to memory are only ever set for
interrupting event queues, not for any other interrupt sources.

By definition there can only be one legacy interrupt handler per
function, so there is no need to worry about detecting a fatal
interrupt more than once.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>