gpio: mvebu: fix blink counter register selection

The blink counter A was always selected because 0 was forced in the
blink select counter register.
The variable 'set' was obviously there to be used as the register value,
selecting the B counter when id==1 and A counter when id==0.

Tested on clearfog-pro (Marvell 88F6828)

Fixes: 757642f9

 ("gpio: mvebu: Add limited PWM support")
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>