diag: Free the buffer for failed entry addition to buf table

Buffer allocated is added to read channel buf table to receive
packets from mhi on callback. Free the allocated buffer for failing
to add an entry of this buffer to the buf table.

Change-Id: Ie24354b6de0a06da1a38d09fe9ea09e9c4700e0a
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>