mfd: ab8500-debugfs: sizeof() mismatch bugfix

Simple pointer error fix to obtain the expected sizeof() result.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Ashok G <ashok.g@stericsson.com>
Reviewed-by: Mattias WALLIN <mattias.wallin@stericsson.com>