mailbox: mchp-ipc-sbi: Add null check for devm_kasprintf()

Add a check to see if devm_kasprintf() is not NULL in
mchp_ipc_get_cluster_aggr_irq(), returning -ENOMEM if the function
failed.

Assisted-by: gkh_clanker_t1000
CC: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Griffin Kroah-Hartman <griffin@kroah.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>