net/af_xdp: fix pointer storage size

net/af_xdp: fix pointer storage size

'uint64_t' is used to hold the pointer, for 32-bits build this
assumption is wrong and giving following build error:

rte_eth_af_xdp.c: In function ‘xdp_umem_configure’:
rte_eth_af_xdp.c:970:15:
    error: cast to pointer from integer of different size
           [-Werror=int-to-pointer-cast]
  970 |   base_addr = (void *)get_base_addr(mb_pool, &align);
      |               ^

Replacing the 'uint64_t' return type of the 'get_base_addr()' to the
'uintptr_t'.
Although not sure if the overall logic supports the 32-bits, using
'uintptr_t' should be safe both for 64/32 bits.

Fixes: d8a210774e1d ("net/af_xdp: support unaligned umem chunks")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Ciara Loftus <ciara.loftus@intel.com>