diag: Do not fail pcie cb registrations for EEXIST return

MHI driver returns -EEXIST value if the channel is open during
registration and no callback will be issued. Prevent diag
pcie callback registration and pcie channel registration by
handling -EEXIST error.

Change-Id: I3d0cffdc00a82cacbe4d26187509f9d650e3990e
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>