Fix unused variables in bn/generic.c

bl and bh are never used by macros if either BN_UMULT_LOHI or
BN_UMULT_HIGH are defined.

Change-Id: I7fdd45014a6b78cc586b5223396e09bc213608a5
Reviewed-on: https://boringssl-review.googlesource.com/2105
Reviewed-by: Adam Langley <agl@google.com>