misc: aspeed-p2a-ctrl: fix mixed declarations

Fix up mixed declarations and code in aspeed_p2a_mmap.

Tested: Verified the build had the error and that this patch resolved it
and there were no other warnings or build errors associated with
compilation of this driver.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>