It is best practice to clear and mask all interrupts before
associating the IRQ, and this should be done after the clock
is enabled.

This corrects a bad result from zynq_fpga_ops_state on bootup
where left over latched values in INT_STS_OFFSET caused it to
report an unconfigured FPGA as configured.

After this change the boot up operating state for an unconfigured
FPGA reports 'unknown'.
Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Acked-by: Alan Tull <atull@opensource.altera.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>