mraa.c: do not check return value of mraa_gpio_dir when used on muxes

Doing so breaks galileo gen2 (gpio2) and others. Failing early is rather
pointless in any case so we may as well attempt the write, muxes do not
neccesarily have directions that work

Fixes #68

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>