staging: r8188eu: add check for kzalloc

As kzalloc() may return null pointer, it should be better to
check the return value and return error if fails in order
to avoid dereference of null pointer.
Moreover, the return value of rtw_alloc_hwxmits() should also
be dealt with.

Fixes: 15865124feed ("staging: r8188eu: introduce new core dir for RTL8188eu driver")
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Link: https://lore.kernel.org/r/20220518075957.514603-1-jiasheng@iscas.ac.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
