Fix bogus pass for syscon_generic introduced in r327621

ian@ pointed out that BUS_PASS_DEFAULT + $anything is bogus, given that
BUS_PASS_DEFAULT is defined as __INT_MAX. Instead, we take a page out of
imx6_usbphy's book and use BUS_PASS_DEFAULT - 1000 to achieve the desired
effect of syscon_generic attaching before if_awg and other potential
consumers, but late enough that more specialized implementations should have
no problem attaching instead.

Reported by: ian