power: supply: core: constify psy_tcd_ops.

File size before:
text	data	bss	dec   hex filename
4240	 200	 80	4520 11a8 drivers/power/supply/power_supply_core.o

File size After adding 'const':
text	data	bss	dec   hex filename
4296	 136	 80	4512 11a0 drivers/power/supply/power_supply_core.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>