staging: most: core: rename struct most_c_aim_obj to pipe

This patch replaces the confusing name of struct "most_c_aim_obj" with
"pipe" to better express the function that is behind the structure.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>