That firmware file which is mentioned in "dmesg" is part of the kernel-firmware package of Slackware 14. Two possible causes for your errors that I can see:
Are you using the Slackware generic kernel with an initial ramdisk? In that case you have to copy the missing firmware file "/lib/firmware/i2400m-fw-usb-1.5.sbcf" from your filesystem into the initrd:
# mkdir /boot/initrd-tree/lib/firmware
# cp -ia /lib/firmware/i2400m-fw-usb-1.5.sbcf /boot/initrd-tree/lib/firmware
Or, if you are not using an initrd, perhaps you forgot to upgrade the kernel-firmware package?