mfd: ab8500-debugfs: Simplify invalid debugfs data checking

Noticed during a coding review, if we reorganised the checking a
little, we can rid the code of a pointless 'else'.  Whilst looking
for this particular code hunk I noticed another pointless 'else',
which I've subsequently fixed in this patch.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>