staging: android: ion: Use ERR_CAST instead of ERR_PTR

Fix the following coccinelle warnings in ion.c:
drivers/staging/android/ion/ion.c:511:9-16: WARNING: ERR_CAST can be used with buffer
drivers/staging/android/ion/ion.c:218:9-16: WARNING: ERR_CAST can be used with table
drivers/staging/android/ion/ion.c:1150:9-16: WARNING: ERR_CAST can be used with dmabuf

Signed-off-by: Iulia Manda <iulia.manda21@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>