bhnd: Correct the softc size in the siba_bhndb_driver definition

struct siba_bhndb_softc embeds struct siba_softc and adds an extra
field, "quirks". In practice, this bug was harmless since "quirks" is
unconditionally initialized during driver attach and would have lived in
the redzone of the softc allocation, but KASAN catches the out-of-bounds
access.

PR: 275515
Reported by: Frank Hilgendorf <frank.hilgendorf@posteo.de>
MFC after: 1 week