x86/platform/pmc_atom: Fix warning when CONFIG_DEBUG_FS=n

When compiling with CONFIG_DEBUG_FS=n, GCC emits an unused
variable warning for pmc_atom.c because "ret" is used only
within the CONFIG_DEBUG_FS block.

This patch adds a dummy #ifdef for pmc_dbgfs_register() when
CONFIG_DEBUG_FS=n to simplify the code and remove the warning.

Signed-off-by: Martin Kelly <martkell@amazon.com>
Acked-by: "Li, Aubrey" <aubrey.li@linux.intel.com>
Cc: vishwesh.m.rudramuni@intel.com
Link: http://lkml.kernel.org/r/1410963476-8360-1-git-send-email-martin@martingkelly.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>