iwlagn: fix *_UCODE_API_MAX output in the firmware field

Currently (3.0-rc2), modinfo iwlagn shows:
    firmware:       iwlwifi-5150-IWL5150_UCODE_API_MAX.ucode
    firmware:       iwlwifi-5000-IWL5000_UCODE_API_MAX.ucode
    firmware:       iwlwifi-6000g2b-IWL6000G2_UCODE_API_MAX.ucode
    firmware:       iwlwifi-6000g2a-IWL6000G2_UCODE_API_MAX.ucode
    firmware:       iwlwifi-6050-IWL6050_UCODE_API_MAX.ucode
    firmware:       iwlwifi-6000-IWL6000_UCODE_API_MAX.ucode
    firmware:       iwlwifi-100-IWL100_UCODE_API_MAX.ucode
    firmware:       iwlwifi-1000-IWL1000_UCODE_API_MAX.ucode
    firmware:       iwlwifi-105-IWL105_UCODE_API_MAX.ucode
    firmware:       iwlwifi-2030-IWL2030_UCODE_API_MAX.ucode
    firmware:       iwlwifi-2000-IWL2000_UCODE_API_MAX.ucode

which is obviously wrong, the user should not see the *_UCODE_API_MAX
macros but the actual ucode API versions here.

The problem are the
    #define *_MODULE_FIRMWARE(api) *_FW_PRE #api ".ucode"
which do not expand api correctly (because this is a macro itself).

Fixed by using __stringify() from linux/stringify.h.

Further information about macro stringification can be found here:
    http://gcc.gnu.org/onlinedocs/cpp/Stringification.html

Signed-off-by: Evgeni Golov <sargentd@die-welt.net>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>