bhnd(4): Fix regression in BCM4331 SPROM pin reference counting.

In r304870, refcount handling was lifted out into a common OTP/SPROM code
path, but the refcount assertions in chipc_disable_sprom_pins() were not
updated accordingly; this triggered an assertion on BCM4331 devices when
releasing a SPROM pin reservation.

Approved by: adrian (mentor, implicit)