wandboard: fix variable name so PXE boot works

All boards that support PXE booting use the pxefile_addr_r variable. Standardise
wandboard with this variable as pxe_addr_r isn't used anywhere else so it's a
typo.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Acked-by: Fabio Estevam <fabio.estevam@nxp.com>