crypto: chtls - use 64-bit arithmetic instead of 32-bit

Cast *val* to u64 in order to give the compiler complete
information about the proper arithmetic to use.

Notice that such variable is used in a context that expects an
expression of type u64 (64 bits, unsigned) and the following
expression is currently being evaluated using 32-bit arithmetic:

val << bit_pos

Addresses-Coverity-ID: 1467425 ("Unintentional integer overflow")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>