MMCCAM: fix a panic after cam_sim_alloc_dev() removal in sdhci.c

During the removal of cam_sim_alloc_dev() in
aeb04e88f51a706ef4b6a380bf5e82d15203fb6a for sdhci.c and the
follow-up build-fix in a72af82e3169fcacfedf9047120679300a4296f8
slot->dev and slot->bus got mixed up for MMCCAM; slot->dev is
only used in the !MMCCAM case so is uninitialised here leading to
a panic; switch back to slot->bus to return to the status quo.

Reviewed by: imp (ack on arm@)
X-Differential Revision: https://reviews.freebsd.org/D30857