iwlwifi: add debugfs to send host command

Add debugfs to send host command in mvm and fmac op modes.
Allows to send host command at runtime via send_hcmd debugfs file.
The command is received as a string that represents hex values.

The struct of the command is as follows:
[cmd_id][flags][length][data]
cmd_id and flags are 8 chars long each.
length is 4 chars long.
data is length * 2 chars long.

Signed-off-by: Shahar S Matityahu <shahar.s.matityahu@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>