crypto: caam - fix build warning when DEBUG_FS not configured

drivers/crypto/caam/ctrl.c: In function 'caam_probe':
drivers/crypto/caam/ctrl.c:55:23: warning: unused variable 'perfmon'

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>