mmc: mmc_getcd/getwp: use sensible defaults

Let mmc_getcd() return true and mmc_getwp() false if mmc driver doesn't
provide handlers for them.

Signed-off-by: Peter Korsgaard <peter.korsgaard@barco.com>
[trini: Add braces around first if test in each case to fix warning]
Signed-off-by: Tom Rini <trini@ti.com>