net: Fix stack info collector in net shell

The cc2520 driver RX thread did not like the stack information
collection macros defined in net_core.h. Both the cc2520 TX and
RX got the same variable name which did not compile.
This is now changed so that the first parameter is added to
the variable name to create a unique variable.

Change-Id: Ia41d01a71afd73af2ef31aa5f7a890a3cf0385aa
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>