Input: iqs7222 - protect against undefined slider size

Select variants of silicon do not define a default slider size, in
which case the size must be specified in the device tree. If it is
not, the axis's maximum value is reported as 65535 due to unsigned
integer overflow.

To solve this problem, move the existing zero-check outside of the
conditional block that checks whether the property is present.

Fixes: e505edae ("Input: add support for Azoteq IQS7222A/B/C")
Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Link: https://lore.kernel.org/r/Y1SRXEi7XMlncDWk@nixie71


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>