staging: r8188eu: simplify update_sta_support_rate params

staging: r8188eu: simplify update_sta_support_rate params

The update_sta_support_rate function takes a pointer to information
elements and the total length of all these elements. Use our variables
for pointer and length instead of calculating them manually.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20221126160129.178697-7-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>