Calibration uses a lot of internal variables, and it used to overflow
even a 3kB stack (by a little amount). This patch shares a temporary
buffer between two functions, so it takes 128 bytes less of stack, and
3kB fit for the wr-nic release.

The problem must still be studied, to reduce stack use.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>