bna: Replace large udelay() with mdelay()

udelay() does not work on some architectures for values above
2000, in particular on ARM:

ERROR: "__bad_udelay" [drivers/net/ethernet/brocade/bna/bna.ko] undefined!

Reported-by: Vagrant Cascadian <vagrant@debian.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>