serial: 8250: Explain the role of @read_status_mask

serial: 8250: Explain the role of @read_status_mask

The role of @read_status_mask has changed over time and seems
to still cause confusion. This can be expected since there is
zero documentation about this driver-specific variable.

Add comments to the initialization of @read_status_mask to
clarify its role.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/20241216171244.12783-5-john.ogness@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>