mfd: stmpe: Rid variable length array Sparse warnings

Numbers are generated by taking the largest currently used values.

drivers/mfd/stmpe.c:252:17:
  warning: Variable length array is used.
drivers/mfd/stmpe.c:857:16:
  warning: Variable length array is used.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>