staging: r8188eu: convert all rtw_zvmalloc calls to vzalloc calls

staging: r8188eu: convert all rtw_zvmalloc calls to vzalloc calls

Convert all rtw_zvmalloc calls within the driver to use the existing
kernel vzalloc function, which has the same semantics. Also rewrite the
two places where it is mentioned in comments to say vzalloc, and remove
the redundant cast to struct adapter * in ./os_dep/usb_intf.c as vzalloc
returns void *.

The reason for the conversion is that rtw_zvmalloc is just a
preprocessor definition for _rtw_zvmalloc which itself is just an inline
wrapper around vmalloc which then zeroes the memory out. As vzalloc does
the same thing via usage of __GFP_ZERO, this code is redundant and can
subsequently be removed.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210818234853.208448-5-phil@philpotter.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>