misc: ad525x_dpot: Fix the enabling of the "otpXen" attributes

Currently writing the attributes with "echo" will result in comparing:
"enabled\n" with "enabled\0" and attribute is always set to false.

Use the sysfs_streq() instead because it treats both NUL and
new-line-then-NUL as equivalent string terminations.

Signed-off-by: Dan Bogdan Nechita <dan.bogdan.nechita@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>