platform/mellanox: mlxbf-pmc: fix signedness bugs

These need to be signed for the error handling to work.  The
mlxbf_pmc_get_event_num() function returns int so int type is correct.

Fixes: 1ae9ffd3

 ("platform/mellanox: mlxbf-pmc: Cleanup signed/unsigned mix-up")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/a4af764e-990b-4ebd-b342-852844374032@moroto.mountain


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>