staging: vt6656: clean error path for firmware management

Avoid discarding return value of functions called during firmware
management process. Handle such return value and return 0 on success or
a negative errno value on error.

Signed-off-by: Quentin Deslandes <quentin.deslandes@itdev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>