Fixing the error "Direct firmware load for isl3887usb failed with error -2"

To use the wusb54ag adapter on raspian you need to load the proper firmare.

Connect the usb device and execute dmesg, you will see this error:

[  591.757432] usb 1-1.5: new high-speed USB device number 14 using dwc_otg
[  591.892857] usb 1-1.5: New USB device found, idVendor=13b1, idProduct=000c
[  591.893054] usb 1-1.5: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[  591.893068] usb 1-1.5: Product: Wireless A/G USB Network Adapter
[  591.893079] usb 1-1.5: Manufacturer: Cisco Linksys
[  591.997446] usb 1-1.5: reset high-speed USB device number 14 using dwc_otg
[  592.128448] usb 1-1.5: Loading firmware file isl3887usb
[  592.130457] usb 1-1.5: Direct firmware load for isl3887usb failed with error -2
[  592.130538] usb 1-1.5: Firmware not found.
[  592.130551] usb 1-1.5: failed to initialize device (-2)

Sometimes you will see:

[15929.205815] usb 1-1.3: failed to initialize device (-2)

To fix the error download the firmware to the /lib/firmware path, example:

cd /lib/firmware/
sudo wget http://daemonizer.de/prism54/prism54-fw/fw-usb/2.13.25.0.lm87.arm -O isl3887usb
ll isl3887usb

This also applies to the Thomson SPeedtouch 121g Wireless 802.11g USB adapter.