drivers: perf: marvell_cn10k: fix an IS_ERR() vs NULL check

The devm_ioremap() function does not return error pointers.  It returns
NULL.

Fixes: 036a7584 ("drivers: perf: Add LLC-TAD perf counter support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20211217145907.GA16611@kili


Signed-off-by: Will Deacon <will@kernel.org>