pl181_send_command() do a bus transaction (send or receive),
rename it as pl181_do_command().

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200705204630.4133-3-f4bug@amsat.org>