firmware: qcom: scm: Rework dload mode availability check

QCOM_SCM_BOOT_SET_DLOAD_MODE scm command is applicable for very
older SoCs where this command is supported from firmware and
for newer SoCs, dload mode tcsr registers is used for setting
the download mode.

Currently, qcom_scm_set_download_mode() checks for availability
of QCOM_SCM_BOOT_SET_DLOAD_MODE command even for SoCs where this
is not used. Fix this by switching the condition to keep the
command availability check only if dload mode registers are not
available.

Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
Reviewed-by: Elliot Berman <quic_eberman@quicinc.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/1711034642-22860-3-git-send-email-quic_mojha@quicinc.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>