iwlagn: separate and enhance the fixed rate from

For testing purpose, we need better control of msc from user application.
Separate the fixed_rate between debugfs and testmode and enforce it.

Signed-off-by: Kenny Hsu <kenny.hsu@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>