rockusb: modify READ_CAPACITY handling; use scsi_eh_prep_cmnd

[CFSworks]

This is the "proper" way to do this, vs. the old hacky approach of switching out the scatter-gather list.

Fixes a bug where the size is not properly reported by the driver.

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To test this PR:

• On current released BMC firmware, put some RK1 node into msd mode (tpi advanced -n 1 msd)
• Ensure the rockusb driver is loaded (lsmod; then modprobe rockusb if necessary)
• Check the size of the RK1 eMMC, reported in 512-byte sectors (cat /sys/class/block/sda/size)
• Convert to GiB with the formula: n*512/1024/1024/1024, observe wildly incorrect value
• Apply patch, rebuild firmware (or simply replace rockusb.ko file), and repeat above test; observe correct size value

[CFSworks]

For reference, the correct eMMC size values (as determined from /sys/class/block/mmcblk0/size on the RK1 itself, not from the BMC):

• RK1.0: 30777344
• RK1.2: 61071360