staging: r8188eu: Replace zero-length array with flexible-array member

One-element and zero-length arrays are deprecated and should be
replaced with flexible-array members[1].

Replace zero-length array with flexible-array member and make use
of the struct_size() helper in kmalloc().

[1] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/78
Link: https://github.com/KSPP/linux/issues/160


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210929193658.GA339070@embeddedor


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