for unknown reasons, multiwrite is busted in usdhc:
sdhc: write error intr 10 stat ff88858e
usdhccmd: need to reset Datinhibit intr 10 stat ff88858e
usdhc: cmd 193a0027 arg 1e5b6b error intr 18010 stat ff88858f
i'm disabling it for now, adding a flag to the SDio struct.
sdbio() tests if it can pass the buffer pointer directly to
the driver when it is already in kernel memory. we also need
to check if the buffer is properly aligned but alignment
requirement is handled in system specific sdmalloc() and
was not known to devsd.
to solve this, we *always* page align sd buffers and get rid
of the system specific sdmalloc() macro (was only used in bcm
kernel).
