iwlwifi: add RATE_MCS_RATE_MSK

Throughout the code we use rate_n_flags & 0xff to extract the lower byte
of the rate_n_flags u32 that contains the information about the rate.
Add a #define and remove the use of the magic number.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>