mrsas: Don't leak a stack pointer value in the softc.

mrsas_issue_blocked_cmd stores a pointer to an on-stack variable
in its softc so that the driver can call wakeup() on the correct
pointer. Once the loop around tsleep() has finished however, the
pointer is no longer needed and any further use would be invalid.
Clear sc->chan to NULL after the loop.

Reported by: GCC -Wdangling-pointer
Differential Revision: https://reviews.freebsd.org/D37628