staging: r8188eu: Make firmware buffer persistent

The present code reloads the firmware file from the disk every time the interface
re-inits. Change to hold the firmware in memory, and only download to the
device.

Signed-off-by: Stas Sergeev <stsp@users.sourceforge.net>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>