gpio: pcal9535a: use i2c_poll_transfer() for register read

The newly introduced i2c_poll_transfer() is now used for register
read for PCAL9535A GPIO driver. The old write then read does not
work because every read or write ends with STOP. However,
the chip requires RESTART after writing the register address.
So the new generic transfer function is perfect for this.

Change-Id: I56d7ebe08f68cb04731c72138d60645ef124f65e
Signed-off-by: Daniel Leung <daniel.leung@intel.com>