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

(cherry picked from commit 4c3aa00c0a0093c78f42d138bb9eef9b1a7cbb39)