Merge remote-tracking branch 'remotes/philmd-gitlab/tags/sd-next-20201026' into staging

SD/MMC patches

- Fix a bug in CMD6/SWITCH_FUNCTION (Bin Meng)
- Minor housekeeping patches

CI jobs result:
. https://cirrus-ci.com/build/5461987880599552
. https://gitlab.com/philmd/qemu/-/pipelines/207532287
. https://travis-ci.org/github/philmd/qemu/builds/738901111
. https://app.shippable.com/github/philmd/qemu/runs/888/summary/console

# gpg: Signature made Mon 26 Oct 2020 11:02:22 GMT
# gpg:                using RSA key FAABE75E12917221DCFD6BB2E3E32C2CDEADC0DE
# gpg: Good signature from "Philippe Mathieu-Daudé (F4BUG) <f4bug@amsat.org>" [full]
# Primary key fingerprint: FAAB E75E 1291 7221 DCFD  6BB2 E3E3 2C2C DEAD C0DE

* remotes/philmd-gitlab/tags/sd-next-20201026:
  hw/sd/sdcard: Zero out function selection fields before being populated
  hw/sd/sdcard: Make iolen unsigned
  hw/sd/sdcard: Constify sd_crc*()'s message argument
  hw/sd/sdcard: Simplify cmd_valid_while_locked()
  hw/sd/sdcard: Update the SDState documentation

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>