plugin/ssa_extract.c: Eliminate compilation warnings (if compiled with -Wunused-but-set-variable)

The following compilation warnings are eliminated:

src/ssa_extract.c: In function âssa_db_extract_switch_portâ:
src/ssa_extract.c:454: warning: variable âmax_pkeysâ set but not used
src/ssa_extract.c: In function âssa_db_extract_host_portâ:
src/ssa_extract.c:521: warning: variable âmax_pkeysâ set but not used

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>