mfd: ucb1x00-core: Rewrite ucb1x00_add_dev()

Error handling is on-its-head in this function. After invoking a function we
should examine the return code and return the error value if there was one.
Instead, this function checks for success and goes onto provide functionality
if success was received. Not so bad in a simple function like this, but in
a more complex one this could end up drowning in curly brackets.

Signed-off-by: Lee Jones <lee.jones@linaro.org>