mmc: test: remove ambiguity in test description

When reading the test description, I thought a correction of the
xfer_size was tested, which is not the case. It is tested that the
xfer_size is correct. Use 'proper xfer_size' to remove this ambiguity.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200817115838.2981-1-wsa@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>