Fix errneous checks for OF_finddevice(9) return value

[gonzo]

Summary

OF_finddevices returns -1 in case of failure. Some code in
existing drivers checked return value to be equal to 0 or
less/equal to 0. Most of these checks were for negative
cases that were never triggered so trhere was no negative
impact on functionality.

This patch fixes all instances of such erroneous checks.

[nwhitehorn]

Looks good. Thanks for doing this!