ar71xx: make IRQ fixes target specific
Move the IRQ fix from generic to ar71xx specific.
Other targets like ath79 have specific pathes to delete this code.
This resulted in a build failure on ath79
Fixes: 00d48bcac08a ("ar71xx: Fix potentially missed IRQ handling during
dispatch")
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>