Correctly set max transfer size as 64K

[kibab]

See https://reviews.freebsd.org/D12762, comment from Marius:

Side-note: Prior to UHS-II, the maximum transfer size for SDHCI is limited by the 16-bit block count register (that maximum may even shrink further depending on the re-tuning mode a particular SDHCI controller implements, though). So for !MMCCAM, shdci(4) correctly hardcodes 65535 for MMCBR_IVAR_MAX_DATA in the !re-tuning case. However, in case of MMCCAM, shdci(4) currently incorrectly sets cpi->maxio to MAXPHYS for XPT_PATH_INQ.