gpioctl: fix compiler warning
gpioctl: fix compiler warning

This fixes
warning: 'rv' may be used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Michael Heimpold <59934addbc17c0fa64a0bf4bb38537fc23439974@heimpold.de>

mhei/libugpio,mhei/libugpio