omap_intc: convert ffs(3) to ctz32() in omap_inth_sir_update()

Rewrite the loop using level &= level - 1 to clear the least significant
bit after each iteration.  This simplifies the loop and makes it easy to
replace ffs(3) with ctz32().

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1427124571-28598-8-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
