MIPS: pci-mt7620: fix PLL lock check

MIPS: pci-mt7620: fix PLL lock check

Upstream a long-standing OpenWrt patch [0] that fixes MT7620 PCIe PLL
lock check. The existing code checks the wrong register bit: PPLL_SW_SET
is not defined in PPLL_CFG1 and bit 31 of PPLL_CFG1 is marked as reserved
in the MT7620 Programming Guide. The correct bit to check for PLL lock
is PPLL_LD (bit 23).

Also reword the error message for clarity.

Without this change it is unlikely that this driver ever worked with
mainline kernel.

[0]: https://lists.infradead.org/pipermail/lede-commits/2017-July/004441.html

Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-mediatek@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>