sdcardio: fix card size for SDXC cards > 32 GB

[mikeysklar]

Backport of #11012 to 10.2.x.

SDXC cards larger than 32 GB were reporting roughly half their actual block count
over USB MSC. The CSD v2.0 C_SIZE field is 22 bits but only the bottom 16 were
being read — the fix reads all 22.

Test results — Metro RP2350, 64 GB exFAT SDXC

	Before	After
macOS disk size	28.9 GB	63.3 GB ✓
Linux disk size	26.9 GiB	58.9 GiB ✓

Regression tested with a 16 GB FAT32 SDHC card — size correct on both platforms,
no impact on cards ≤ 32 GB.

Before:

$ lsblk -o NAME,SIZE,FSTYPE,LABEL
sdc     26.9G
└─sdc1  26.9G exfat    SDXC

After:

$ lsblk -o NAME,SIZE,FSTYPE,LABEL
sdc     58.9G
└─sdc1  58.9G exfat    SDXC

[mikeysklar]

Backport of #11012 (main).