staging: gdm72xx: remove completed TODO item

Remove an item from the TODO file since it appears to have been
completed by the following commits:
* 1839c7eb "staging/gdm72xx: usb_boot: replace firmware upgrade API"
* 3afcb91c "staging/gdm72xx: usb_boot: replace firmware upgrade API in
  em_download"
* 9e412a0a "staging/gdm72xx: sdio_boot: replace firmware upgrade API"
Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>

drivers/staging/gdm72xx/TODO

 TODO:
 - Replace kernel_thread with kthread in gdm_usb.c
-- Replace hard-coded firmware paths with request_firmware in
-  sdio_boot.c and usb_boot.c
 - Clean up coding style to meet kernel standard.