mmc: mmc_test: Replace hard-coded values with macros and consolidate test parameters

mmc: mmc_test: Replace hard-coded values with macros and consolidate test parameters

Replacing hard-coded values with standardized macros to improve code clarity,
simplify future maintenance.

Meanwhile, introduce global bs and sg_len arrays for block sizes and SG lengths,
eliminating redundant local definitions in multiple test functions.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>