Errors when building device capabilities

[maitredede]

Hi,

When trying to build device capabilities map, errors are raised for some arguments. With the commits that handle errors, when trying to open a device, errors are always raised, so my device list becomes empty...

Can be related to #14 and #15 .

[maitredede]

I have added some logging : https://github.com/MaitreDede/golang-evdev/tree/err-checks

Here is what I get with a keyboard :

2019/08/30 13:52:07 setDeviceCapabilities().ioctl(EVIOCGBIT(evtype, KEY_MAX)) evtype=20 error: invalid argument
2019/08/30 13:52:07 setDeviceCapabilities().ioctl(EVIOCGBIT(evtype, KEY_MAX)) evtype=20 error: invalid argument