staging: r8188eu: simplify complex pointer casting

staging: r8188eu: simplify complex pointer casting

Pointers to structures udphdr and dhcpMessage are derived by casting
adjacent pointers with size_t. Such typecast of pointer using size_t
is not preferred. The code looks complex and delicate. Simplify such
casting by utilizing generic "void *" casting.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Deepak R Varma <drv@mailo.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/1428580d09a9916899209c9278dca40ee2d297d3.1667755987.git.drv@mailo.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>