mfd: rave-sp: Remove VLA

As part of the effort to remove VLAs from the kernel[1], this creates
constants for the checksum lengths of CCITT and 8B2C and changes
crc_calculated to be the maximum size of a checksum.

https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Kyle Spiers <ksspiers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>