staging: ks7010: rename identifier rc to ret

Driver uses identifier 'rc' to hold the value for error return
code. The rest of the driver predominately uses 'ret' for this
purpose. It is easier to follow the code if one name is used for one
task.

Rename identifier 'rc' to 'ret'.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>