Fix erroneous boolean expression conversions

In fb90157c9ad7f54c4012354d63bb8bebe259abb1, asserts were introduced
changing the error checking style in large portions of the code base
from `if(error_condition) fail();` to `assert(!error_condition);`. In
doing so, not all boolean conditions were negated properly. This commit
restores the original semantics of the error checks as they were before
fb90157, (unless an error check has been changed upstream, in which
case it is ignored).

The practical effects of this commit is that it should restore proper
I2C and SPI functionality on the LPC15XX and nRF51822, respectively.