mfd: ab8500-debugfs: Prevent initialised field from being over-written

Due to the lack of parity in the way array fields have been named/
numbered, a mistake was made where more debug fields were declared
than actually existed.  In doing so, 2 fields were added, which
although unclear, were already declared in the array.  The result
was that the latter declarations trashed the former ones.

This patch places the array back in the correct order and removes
the offending NULL entries.

While we're at it, let's ensure this doesn't happen again by naming
each field properly and add a new *_LAST define to describe how
many fields there should be.

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