greybus: gbuf: have caller set actual_length

greybus: gbuf: have caller set actual_length

A Greybus buffer has both a transfer_buffer_size field, which is the
size in bytes of the transfer buffer, and an actual_length field,
which is the number of bytes in that buffer that are actually
consumed.  The user of the buffer--and not the buffer allocation
method--should be setting the actual_length field.

Stop setting the actual length on the es1-ap-usb alloc_gbuf_data
method.  And *do* set it in gb_operation_create(), where we can
presume the operation being allocated will consume all the bytes
requested.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>