Input: ili210x - fix return value of is_visible function

Input: ili210x - fix return value of is_visible function

The is_visible function expects the permissions associated with an
attribute of the sysfs group or 0 if an attribute is not visible.

Change the code to return the attribute permissions when the attribute
should be visible which resolves the warning:

  Attribute calibrate: Invalid permissions 01

Fixes: 15d4bac1794a ("Input: ili210x - optionally show calibrate sysfs attribute")
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Sven Van Asbroeck <TheSven73@gmail.com>
Link: https://lore.kernel.org/r/20200209145628.649409-1-luca@z3ntu.xyz
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>