gpio/tca6424: merge I2C transactions, remove cast

This is a follow-up to ae79c190

 "[PATCH v2] Add support for TCA6424"
merged in the v3.5 merge window.  It fixes comments made when the
patch was merged.
- Use 3 byte transfers instead of two separate transfers (2+1 byte)
- An unnecessary cast removed

Signed-off-by: Andreas Schallenberg <Andreas.Schallenberg@3alitytechnica.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>