perf: arm-cmn: Fix unsigned comparison to less than zero

Ensure that the 'irq' field of 'struct arm_cmn_dtc' is a signed int
so that it can be compared '< 0'.

Link: https://lore.kernel.org/r/20200929170835.GA15956@embeddedor


Addresses-Coverity-ID: 1497488 ("Unsigned compared against 0")
Fixes: 0ba64770 ("perf: Add Arm CMN-600 PMU driver")
Reported-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>