fix crash with LOCKDEBUG+DIAGNOSTIC kernels:

sdhc_bus_clock() calls mutex_enter().
then in DIAGNOSTIC section, sdhc_bus_clock() calls sdhc_card_detect().
sdhc_card_detect() calls mutex_enter() on the same mutex again.
when LOCKDEBUG is enabled, you get

Mutex error: lockdebug_wantlock: locking against myself

(cegger)