* get_freq_grid() already maps GridSpacing code 8 to 150GHz, but
  get_laser_config_freq() and set_laser_freq() were never updated to
  match: get_laser_config_freq() fell through to the generic branch
  and computed channel * 150 instead of channel * 25, and
  set_laser_freq() had no grid==150 case at all and returned False
  for any 150GHz-only module.
* get_laser_config_freq(): treat grid 150 like grid 75 (channel step
  is 25GHz for both; 150GHz channels are simply every 6th 25GHz step).
* set_laser_freq(): add a grid==150 branch, GridSpacingTx1 code 0x80,
  channel number required to be a multiple of 6.

Signed-off-by: Grigory Solovyev <gs1571@gmail.com>

* OIF-CMIS 5.3 Table 8-66 defines the 150GHz grid as
  Frequency = 193.1 + (n+3) x 0.025 THz, unlike the 75GHz grid
  (Frequency = 193.1 + n x 0.025 THz) which has no additive offset.
  get_laser_config_freq()/set_laser_freq() treated both grids identically,
  so every 150GHz frequency was off by exactly +75GHz (3 channels x 25GHz)
  from what was requested.
* get_laser_config_freq() now adds 3 to the channel before scaling for
  grid 150; set_laser_freq() subtracts 3 from the computed channel number
  before the "multiple of 6" validation and before writing it to the
  module.
* Add/update tests in test_ccmis.py covering the corrected 150GHz
  formula and asserting the exact channel number written to the module.

Signed-off-by: Grigory Solovyev <gs1571@gmail.com>