mmc-fdt: fix mmc_fdt_gpio_get_{present,readonly}

Currently, mmc_fdt_gpio_get_{present,readonly} return all time true.
true ^ 100b = true
false ^ 100b = true
since that's done after promotion to integers. Use !! to convert
the bit to a bool before xor.

Reviewed by: imp@ (converted to (bool) to !! for portability)
Pull Request: https://github.com/freebsd/freebsd-src/pull/461