n_tty: do only one cp dereference in n_tty_receive_buf_standard

It might be confusing for readers: there are three distinct dereferences
and increments of 'cp' in n_tty_receive_buf_standard. Do it on a single
place, along with/before the 'fp' dereference.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-9-jslaby@suse.cz


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