mhi: mhi_uci: Create char dev only after PCIe device ID is known

MHI host may not have PCIe device info during channel
registration. Defer creation of char device until
PCIe bus enumeration is completed.

CRs-Fixed: 2040899
Change-Id: I6372523bd6a758f29dad09d207b86c3f87e2f2cc
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>