usb: typec: ucsi: Fix NULL pointer dereference

Making sure the UCSI debugfs entry actually exists before
attempting to remove it.

Fixes: df0383ffad64 ("usb: typec: ucsi: Add debugfs for ucsi commands")
Reported-by: Dave Hansen <dave.hansen@intel.com>
Closes: https://lore.kernel.org/linux-usb/700df3c4-2f6c-85f9-6c61-065bc5b2db3a@intel.com/
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Suggested-by: Mario Limonciello <mario.limonciello@amd.com>
Cc: Saranya Gopal <saranya.gopal@intel.com>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Link: https://lore.kernel.org/r/20230906084842.1922052-1-heikki.krogerus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

usb: typec: ucsi: Add debugfs for ucsi commands

Add support for UCSI commands through the following debugfs:
# /sys/kernel/debug/usb/ucsi/$UCSI_DEVICE/command
# /sys/kernel/debug/usb/ucsi/$UCSI_DEVICE/response

Eg: To execute UCSI GetCapabilities:
# echo 0x6 > /sys/kernel/debug/usb/ucsi/<ucsi device>/command
Then read the result,
# cat /sys/kernel/debug/usb/ucsi/<ucsi device>/response
0x02000320000000020000ff0400000445

UCSI command will be written into the command file and the
response for the command can be viewed under the response file.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Saranya Gopal <saranya.gopal@intel.com>
Co-developed-by: Rajaram Regupathy <rajaram.regupathy@intel.com>
Signed-off-by: Rajaram Regupathy <rajaram.regupathy@intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20230807105205.742819-1-saranya.gopal@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>