serial: sh-sci: describe locking requirements for invalidating RXDMA

Make sure everyone knows that calling this function needs protection.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20240506114016.30498-8-wsa+renesas@sang-engineering.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>