sdhci: Defer XPT_SET_TRAN_SETTINGS to the slot taskqueue

[manu]

Summary

Only XPT_MMC_SET_TRAN_SETTINGS was handed to the slot taskqueue.
XPT_SET_TRAN_SETTINGS, which mmc_xpt.c issues too, still called
sdhci_cam_settran_settings() directly from the CAM done thread with the
CAM device lock held. Changing the bus clock can sleep (clk_set_freq()
takes the clock topology sx lock), which panics with "sleeping thread
holds CAM device lock".

Handle both opcodes the same way and complete the request from the
taskqueue.