staging: rtl8712/: Using comparison to true is error prone

clear below issues reported by checkpatch.pl:

CHECK: Using comparison to true is error prone
CHECK: Comparison to NULL should be written "!oldest"

Signed-off-by: John Oldman <john.oldman@polehill.co.uk>
Link: https://lore.kernel.org/r/20200710113113.1648-1-john.oldman@polehill.co.uk


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