staging: ks7010: rename return value identifier

Driver uses multiple identifier names for the same task (retval, ret,
rc). It would be easier to read the code if a single task is
identified with a single name. 'ret' is the most common return value
identifier name found in the kernel tree, following the principle of
least surprise using 'ret' is a decent choice.

Rename rc -> ret
Rename retval -> ret

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