net: qrtr: mhi: check whether ul pkt list is empty

list_first_entry should not be called for empty list.

Add check for empty list before calling list_first_entry.

CRs-Fixed: 2596626
Change-Id: Ie6be42f6aa4915e1495b87faf5a0d933086b9d01
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>