gpio: omap: simplify set_multiple()

One of the reasons for set_multiple() to exist is to allow multiple
GPIOs on the same chip to be changed simultaneously - see commit
5f424243 ("gpiolib: allow simultaneous setting of multiple GPIO
outputs"):

 - Simultaneous glitch-free setting of multiple pins on any kind of
   parallel bus attached to GPIOs provided they all reside on the
   same chip and bank.

In order for this to work, we should not use the atomic set/clear
registers, but instead read-modify-write the dataout register.  We
already take the spinlock to ensure that happens atomically, so
move the code into the set_multiple() function and kill the two
helper functions.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>