staging: fix mei build when PM is not enabled

Fix mei build when CONFIG_PM is not enabled (i.e., fix typo):

drivers/staging/mei/main.c:1159: error: 'MEI_PM_OPS' undeclared here (not in a function)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>