staging: rtl8188eu: os_dep: Remove useless cast on kzalloc

Casting rhe return value which is a void pointer is redundant.
The conversion from void pointer to any other pointer type is
guaranteed by the C programming language, as mentioned in
Documentation/CodingStyle, Chap 14.

Removed the cast on kzalloc return value.

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>