net/qede: fix compilation with -Og

net/qede: fix compilation with -Og

The compilation with gcc-6.3.0 and EXTRA_CFLAGS=-Og gives the following
error:

  CC qede_rxtx.o
  qede_rxtx.c: In function ‘qede_start_queues’:
  qede_rxtx.c:797:9: error: ‘rc’ may be used uninitialized in this
                            function [-Werror=maybe-uninitialized]
    return rc;
           ^~

If there is no Rx or Tx queue, rc will not be initialized. Fix it
by initializing rc to -1.

Fixes: 4c4bdadfa9e7 ("net/qede: refactoring multi-queue implementation")
Cc: stable@dpdk.org
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Harish Patil <harish.patil@cavium.com>