serial: core: Revert checks for tx runtime PM state

This reverts commit 81a61051.

With tty and serdev controller moved to be children of the serial core
port device, runtime PM usage count of the serdev controller now
propagates to the serial hardware controller parent device as expected.

Cc: Maximilian Luz <luzmaximilian@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20231113080758.30346-2-tony@atomide.com


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