usb/misc: usb4604: Use pm_ptr() macro

Use the newly introduced pm_ptr() macro, and mark the suspend/resume
functions __maybe_unused. These functions can then be moved outside the
CONFIG_PM_SUSPEND block, and the compiler can then process them and
detect build failures independently of the config. If unused, they will
simply be discarded by the compiler.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20200903112554.34263-9-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>