USB: serial: keyspan_pda: drop unused firmware reset status

Drop the unused firmware reset status which would already have been
logged.

This suppresses the corresponding W=1 (-Wunused-but-set-variable)
warning.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>