cmd: pxe: Fix bootm argument count

pxe command parses the init ramfs address(through initrd lable in
pxe config file), but is not passing it to bootm command as argument
as bootm_argc count is not increased.

Signed-off-by: Abhishek Shah <abhishek.shah@broadcom.com>
Signed-off-by: Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>
Reviewed-by: Simon Glass <sjg@chromium.org>