diag: Call diagmem_exit only if the mempool is initialized

Call diagmem_exit in diag_mhi_exit only if the memory pool is
initialized earlier. If diag bridge initialization fails
allow diag apps to still communicate to other processors
without unloading the diag driver.

Change-Id: I0eb8b9a357f172984612175d1b03dd872df91b6f
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>