iwlwifi: make initial calibration set configurable

This patch adds ability to configure initial calibration set. Not all HW
supported by iwlwifi use the same calibration set, XTAL is one example.
Some clean ups are also included in this patch.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>