net/ring: rename map file to standardized name

The filenames of the linker map files for DPDK PMDs, all follow a
standard format: rte_pmd_<libname>_version.map. The ring version, however,
had eth instead of pmd in the name, so was non-standard. By changing
this, we no longer need the build system to explicitly be given the name of
the mapfile, as it can determine it from the directory name.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>