mfd: Fix debugfs code in AB3100 OTP driver

This fixes the aged and unreachable debugfs code in the AB3100
OTP driver so that it's (A) reachable and (B) works. Bug detected
in parallell by Christoph Egger using VAMOS and Robert P. J. Day
by his kernel scanning script.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Cc: Christoph Egger <siccegge@stud.informatik.uni-erlangen.de>
Cc: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>