libusbg: Allow to set all gadget attrs using one function.

Having specialized functions for each attribute may be
sometime inconvenient to use. Provide also function
whihc allows to set attribute selected by parameter.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
libusbg: Return suitable error codes instead of -1

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
libusbg: Make usbg_lookup_function_type() publicly available

Users of library may often receive function type as a string.
Instead of implementing parsing of such string in each program
it would be more convenient to provide such support in library.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
libusbg: Add own name to AUTHORS file.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
libusbg: Don't print errno when it has not been directly set

Replace ERRORNO() macro with ERROR(). Difference betwen
them is that ERRORNO() prints also standard error string
based on errno value. This variable should not be accessed
when called function doesn't directly set it.

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