ag71xx: replace delay with sleep calls

I don't see that we're in an atomic context so there's no need to
busy-wait. Therefore replace the delay with sleep calls.
See also Documentation/timers/timers-howto.txt. It states:
"In general, use of mdelay is discouraged and code should
be refactored to allow for the use of msleep."

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43539 3c298f89-4303-0410-b956-a3cf2f4a3e73