tty: serial: pch_uart.c: remove debugfs dentry pointer

tty: serial: pch_uart.c: remove debugfs dentry pointer

No need to keep around the dentry pointer for the debugfs file if all it
is used for is to remove it when we are wanting to clean up, as the
pointer can be looked up directly from debugfs instead.

This also removes pointless #ifdef CONFIG_DEBUG_FS brackets as the
compiler is smart enough to handle this properly if debugfs is disabled
without us having to worry about it.

Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: linux-serial@vger.kernel.org
Link: https://lore.kernel.org/r/20210216145900.3835160-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>