mfd: ab8500-debugfs: Remove use of seq_printf return value

The seq_printf return value, because it's frequently misused,
will eventually be converted to void.

See: commit 1f33c41c ("seq_file: Rename seq_overflow() to
     seq_has_overflowed() and make public")
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>