mmc_da: create disk(9) for pre-2.0 SD cards

It does not look like there is anything in mmc_da code that actually
requires protocol 2.0 or later. dev/mmc code also does not have such a
restriction.

Tested with a very old 2GB mini-SD card. Prior to this change mmc_da
would claim the card but would not expose it to GEOM.

Without MMCCAM:
mmc0: <MMC/SD bus> on sdhci_pci0
mmc0: Probing bus
mmc0: SD probe: OK (OCR: 0x00ff8000)
mmc0: Current OCR: 0x00ff8000
mmc0: CMD8 failed, RESULT: 1
mmc0: Probing cards
mmc0: New card detected (CID 1c53565344432020100002982e007600)
mmc0: New card detected (CSD 005e00325f5a83d02db7ffbf96800000)
mmc0: Card at relative address 0xb368 added:
mmc0: card: SD SDC 1.0 SN 0002982E MFG 06/2007 by 28 SV
mmc0: quirks: 0
mmc0: bus: 4bit, 50MHz (high speed timing)
mmc0: memory: 3998720 blocks, erase sector 256 blocks
mmc0: setting transfer rate to 50.000MHz (high speed timing)
GEOM: new disk mmcsd0
mmcsd0: 2GB <SD SDC 1.0 SN 0002982E MFG 06/2007 by 28 SV> at mmc0 50.0MHz/4bit/65535-block
mmc0: setting bus width to 4 bits high speed timing

With MMCCAM and this change:
sdda0 at sdhci_slot0 bus 0 scbus2 target 0 lun 0
sdda0: Relative addr: 0000b368
Card features: <Memory>
sdda0: Serial Number 0002982E
sdda0: SD SDC 1.0 SN 0002982E MFG 06/2007 by 28 SV
GEOM: new disk sdda0

(cherry picked from commit 6fd84a627fa066a98777c7ab1e65bdd885407516)