staging: rtl8723bs: use kernel_read() instead of open-coded version

Replace the manual call to f_op->read() under KERNEL_DS with kernel_read().
This also reduces the number of users of the legacy alias get_ds() for
KERNEL_DS.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>