USB: serial: Remove unused s_priv variable

s_priv was set but never used in keyspan_open() and keyspan_close(),
remove it.

This also makes the serial variable in keyspan_open() unused since
it's only use was to set s_priv, so it is also removed.

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>