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>