staging: rtl8712: xmit_linux.c: Remove leading p from variable names

Remove leading p from the names of the following pointer variables:
- padapter
- pxmitpriv
- pnetdev
- pxmitframe.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>