irqchip: gic-v3: Restore enable bit of spi interrupts

While setting enable bit of spi interrupt, there is
chance of enabling spurious interrupt which is by default
disabled for soc. So instead of setting restore the
previous state of enable bit.

Change-Id: Ie6e363f04864fc6e36be83ebd20b19b5e6b45f54
Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>