drivers: misc: Mark functions as static in ad525x_dpot.c

This patch marks the function ad_dpot_add_files() and
ad_dpot_remove_files() as static in ad525x_dpot.c because they are not
used outside this file.

Thus, it also eliminates the following warnings in ad525x_dpot.c:
drivers/misc/ad525x_dpot.c:644:5: warning: no previous prototype for ‘ad_dpot_add_files’ [-Wmissing-prototypes]
drivers/misc/ad525x_dpot.c:669:13: warning: no previous prototype for ‘ad_dpot_remove_files’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>