mmc: meson-gx: set max block count and request size

So far max_blk_count isn't set what results in a default of value 8
to be used (PAGE_SIZE / block size).

Block length field has 9 bits, so set max_blk_count to 2^9-1 = 511.
In addition set max_req_size because max_blk_count is also limited
by max_req_size / block_size.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>