iwlwifi: fix scan_cmd_size allocation

Allocate scan command with dynamic size based on uCode capability
and num of channels.
This isn't an important fix as the previous allocation was always
too large as it added the scan command size but later subtracted
it (which meant it was supposed to be part of the max scan size.)

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>