power_supply: Drop usage of nowarn variant of sysfs_create_link()

The function is not exported to modules, plus we do want to catch anyone
who tries to create complex hierarchy (in that case we'd need to change
'powers' symlink to a directory, probably under a different name to not
break ABI).

This patch fixes the following build error:

 ERROR: "sysfs_create_link_nowarn" [drivers/power/power_supply.ko] undefined!

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>