staging: r8188eu: use sizeof(*pvar) for allocating structs

Use sizeof(*pvar) instead of sizeof(struct var) when allocating memory.
This conforms to Linux kernel coding style, improves readability,
and decreases the opportunity for bugs if the pointer variable type is
changed. Issue found by checkpatch messages of the following format:

CHECK: Prefer kzalloc(sizeof(*pvar)...) over kzalloc(sizeof(struct var)...)

Signed-off-by: Rebecca Mckeever <remckee0@gmail.com>
Link: https://lore.kernel.org/r/YkXh1HBqaHwT38UC@bertie


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