liblitesdcard/sdcard: adjust card-ready timeout

liblitesdcard/sdcard: adjust card-ready timeout

Testing on nexys4ddr and rocket, approximately 12 iterations of the
timeout loop (using `busy_wait(1)`) are needed to receive a "ready"
response from the SDcard, assuming a "warm" reset where the card has
already been previously initialized.

If the SDcard is ejected and re-inserted, or if the board is "cold-reset"
(e.g., reprogrammed via openocd vs. a simple push of the reset button),
it takes approximately 450 iterations before the SDCard responds with a
"ready" message.

In either case, a timeout of 10 is insufficient. This patch increases
the busy-wait to 10, and the timeout loop counter to 128, which should
cover most cases.

Additionally, make a few minor cosmetic improvements.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>