mfd: ab8500-debugfs: Fix incompatible types in comparison expression issue

Smatch reports:

 drivers/mfd/ab8500-debugfs.c:1804:20: error: incompatible types in comparison expression (different type sizes):
 drivers/mfd/ab8500-debugfs.c:1804:20:    unsigned int *
 drivers/mfd/ab8500-debugfs.c:1804:20:    unsigned long *

This is due to mixed types being compared in a min() comparison.  Fix
this by treating values as signed and casting them to the same type
as the receiving variable.

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