remoteproc: qcom_wcnss: Fix on platforms without fallback regulators

remoteproc: qcom_wcnss: Fix on platforms without fallback regulators

Recent change to handle platforms with only single power domain broke
pronto-v3 which requires power domains and doesn't have fallback voltage
regulators in case power domains are missing. Add a check to verify
the number of fallback voltage regulators before using the code which
handles single power domain situation.

Fixes: 65991ea8a6d1 ("remoteproc: qcom_wcnss: Handle platforms with only single power domain")
Signed-off-by: Matti Lehtimäki <matti.lehtimaki@gmail.com>
Tested-by: Luca Weiss <luca.weiss@fairphone.com> # sdm632-fairphone-fp3
Link: https://lore.kernel.org/r/20250511234026.94735-1-matti.lehtimaki@gmail.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>