phy: qcom-qmp-pcie: drop bogus register update

Since commit 0d58280c ("phy: Update PHY power control sequence") the
PHY is powered on before configuring the registers and only the MSM8996
PCIe PHY, which includes the POWER_DOWN_CONTROL register in its PCS
initialisation table, may possibly require a second update afterwards.

To make things worse, the POWER_DOWN_CONTROL register lies at a
different offset on more recent SoCs so that the second update, which
still used a hard-coded offset, would write to an unrelated register
(e.g. a revision-id register on SC8280XP).

As the MSM8996 PCIe PHY is now handled by a separate driver, simply drop
the bogus register update.

Fixes: e4d8b05a ("phy: qcom-qmp: Use proper PWRDOWN offset for sm8150 USB") added support
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>