mailbox: mchp-ipc-sbi: fix uninitialized symbol and other smatch warnings

mailbox: mchp-ipc-sbi: fix uninitialized symbol and other smatch warnings

Fix uninitialized symbol 'hartid' warning in mchp_ipc_cluster_aggr_isr()
by introducing a 'found' flag to track whether the IRQ matches any
online hart. If no match is found, return IRQ_NONE.

Also fix other smatch warnings by removing dead code in
mchp_ipc_startup() and by returning -ENODEV in dev_err_probe() if the
Microchip SBI extension is not found.

Fixes below smatch warnings:
drivers/mailbox/mailbox-mchp-ipc-sbi.c:187 mchp_ipc_cluster_aggr_isr() error: uninitialized symbol 'hartid'.
drivers/mailbox/mailbox-mchp-ipc-sbi.c:324 mchp_ipc_startup() warn: ignoring unreachable code.
drivers/mailbox/mailbox-mchp-ipc-sbi.c:422 mchp_ipc_probe() warn: passing zero to 'dev_err_probe'

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202512171533.CDLdScMY-lkp@intel.com/
Signed-off-by: Valentina Fernandez <valentina.fernandezalanis@microchip.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>