gpio: etraxfs: fix set register flag

BGPIO_F_UNREADABLE_REG_SET is incorrect, since the set register _is_
readable.  What's really required is BGPIO_F_READ_OUTPUT_REG_SET:
reading the set register reads the set output value.
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-etraxfs.c

@@ -140,7 +140,7 @@ static int etraxfs_gpio_probe(struct platform_device *pdev)
 				 NULL,			/* clr */
 				 regs + port->oe,	/* dirout */
 				 NULL,			/* dirin */
-				 BGPIOF_UNREADABLE_REG_SET);
+				 BGPIOF_READ_OUTPUT_REG_SET);
 		if (ret)
 			return ret;