drivers: staging: ion: fix error check in check_valid_vmid

check_valid_vmid makes an incorrect assumption on the return
value of ion_msm_page_pool_alloc causing an incorrect pointer
access later in ion_msm_page_pool_add.

Change-Id: I45dc78978385498366ba729b447ec365a31474d8
Fixes: cae595f4 ("ion: Check for valid vmids for msm system heap")
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>