gw_ventana: Fix unused variable warnings

The variable nfc_pads is only referenced when CONFIG_CMD_NAND is set,
move the existing guard and drop a now redundant comment.  The variable
gwproto_gpio_pads is never referenced, remove it.

Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Tom Rini <trini@konsulko.com>