mmc: core: Simplify card drive strength mask

Card drive strength selection uses a callback to
which a mask of supported drive strengths is passed.
Currently, the bits are checked against the values
in the SD specifications. That is not necessary
because the callback will anyway match the mask
against a valid value. Simplify by taking the mask
as is but still ensuring that the default mandatory
value (type B) is always supported.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>