libusbg: Don't try to set read only attribute

Some of function attributes are read only on configfs, so library
should not try to set them and accept only empty values in functions
which set multiple attributes at single call.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
libusbg: Add missing const qualifiers

Const qualifers should be placed in all functions which
don't need to modify provided buffers to allow caller to
pass literals or pointers to non-modifable space.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
libusbg: Use reentrant functions to avoid static buffers usage

ether_aton() and ether_ntoa() use static buffers which are
overwritten by next call of this function so those functions
should not be used in multithread environment.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>