tty: can327, move overflow test inside can327_ldisc_rx()'s loop

tty: can327, move overflow test inside can327_ldisc_rx()'s loop

The 'count' is going to be unsigned and the 'count >= 0' test would be
always true then. Move the condition to the loop where this is easier to
check.

It looks as is easier to follow after all too.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: Max Staudt <max@enpas.org>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-can@vger.kernel.org
Link: https://lore.kernel.org/r/20230810091510.13006-15-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>