staging: r8188eu: fix_rate is set but not used.

fix_rate in struct adapter is set but not used. Remove the variable and
the private ioctl code to set it.

Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220102175932.89127-4-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>