gpio/pca953x: Fix wrong pointer type

gpio/pca953x: Fix wrong pointer type

pca953x_get_alt_pdata() uses uint16_t* as result type for
of_get_property(), but numeric of values are __be32.

Checking for negative values is bogus because of-property
values are unsigned by definition.
Instead check for proper property size.

v3: - assume big-endian properties
    - check property size
v2: - removed bogus check for negative property values

Signed-off-by: Dirk Eibach <eibach@gdsys.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>