staging: nvec: fix unconditional pm_power_off teardown

staging: nvec: fix unconditional pm_power_off teardown

tegra_nvec_remove() unconditionally sets pm_power_off = NULL, even if
nvec was not the one that registered it. This breaks any other driver
that may have set pm_power_off to its own handler.

Replace the unconditional assignment with a guarded check so that
pm_power_off is only cleared if nvec was the one that set it.

Also remove the stale FIXME comment, as the guard addresses exactly
what it was asking for.

Reviewed-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Alexandru Hossu <hossu.alexandru@gmail.com>
Link: https://patch.msgid.link/20260427081713.3401874-3-hossu.alexandru@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>