sparc32, leon: Do not overwrite previously set irq flow handlers

This is needed because when scan_of_devices finds the GAISLER_GPTIMER
core that corresponds to the SMP "ticker" timer, the previously set
proper irq flow handler gets overwritten with an incorrect one. This
leads to very flaky timer interrupt handling on some hardware. Proper
updates to handlers can still be done using leon_update_virq_handling.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>