cs5535-gpio: handle GPIO regs where higher (clear) bits are set

The default for non-READ_BACK GPIO regs is to have the clear bits set;
this means that our original errata fix was too simplistic.  This
changes it to the following behavior:

 - when setting GPIOs, ignore the higher order bits (they're for
   clearing, we don't need to care about them).

 - when clearing GPIOs, keep all the bits, but unset (via XOR) the
   lower order bit that negates the clear bit that we care about.  That
   is, if we're clearing GPIO 26 (val = 0x04000000), we first XOR what's
   currently in the register with 0x0400 (GPIO 26's SET bit), and then
   OR that with the GPIO 26's CLEAR bit.

Tested-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>