greybus: kill off the last of gbuf.c

Only three functions remain in "gbuf.c".  Move one of them into
"connection.c" and the other two into "operation.c".

Some more cleanup is coming that will further straighten out gbufs
but for now there's no sense in drawing this out any longer.

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