Staging: rtl8712: fix spelling errors

Found by checkpatch:
- s/cacluated/calculated/
- s/convertor/converter/
- s/psudo/pseudo/
- s/halfs/halves/

Signed-off-by: Joseph Wright <rjosephwright@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>