mhi: cntrl: qcom: save POR PCIe link speed during probe

MHI host could transition into system suspend before receiving
any bandwidth change request from device. Without a default link
state, host would switch link state into unsupported state and
cause a PCIe link down. Avoid this unsupported scenario by
explicitly saving the initial link state during probe.

CRs-Fixed: 2436570
Change-Id: Icc5c681937a51eeb4f2df3c95914f924311ca388
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>