acpi: fix sparse const errors

In this case we want a constant pointer to constant chars:

drivers/misc/thinkpad_acpi.c:3824:19: error: Just how const do you want this type to be?

Like the error says.

drivers/misc/thinkpad_acpi.c:3863:19: error: Just how const do you want this type to be?
drivers/misc/thinkpad_acpi.c:3864:19: error: Just how const do you want this type to be?
drivers/misc/thinkpad_acpi.c:3865:19: error: Just how const do you want this type to be?
drivers/misc/thinkpad_acpi.c:3866:19: error: Just how const do you want this type to be?

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>