firmware: ti_sci: Use system_state to determine polling

Commit b9e8a7d9 ("firmware: ti_sci: Switch transport to polled
mode during system suspend") aims to resolve issues with tisci
operations during system suspend operation. However, the system may
enter a no_irq stage in various other usage modes, including power-off
and restart. To determine if polling mode is appropriate, use the
system_state instead.

While at this, drop the unused is_suspending state variable and
related helpers.

Fixes: b9e8a7d9 ("firmware: ti_sci: Switch transport to polled mode during system suspend")
Reported-by: Francesco Dolcini <francesco@dolcini.it>
Reported-by: Wadim Egorov <w.egorov@phytec.de>
Tested-by: Francesco Dolcini <francesco.dolcini@toradex.com> # Toradex Verdin AM62
Link: https://lore.kernel.org/r/20230620130329.4120443-1-nm@ti.com
Closes: https://lore.kernel.org/all/ZGeHMjlnob2GFyHF@francesco-nb.int.toradex.com/


Signed-off-by: Nishanth Menon <nm@ti.com>