crypto: lib/mpi - use min() instead of min_t()

crypto: lib/mpi - use min() instead of min_t()

min_t(unsigned int, a, b) casts an 'unsigned long' to 'unsigned int'.
Use min(a, b) instead as it promotes any 'unsigned int' to 'unsigned long'
and so cannot discard significant bits.

In this case the 'unsigned long' value is small enough that the result
is ok.

Detected by an extra check added to min_t().

Signed-off-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>