serial: pmac_zilog: add sparse context annotation

Add sparse context annotation to the receive handler, which releases and
reacquires the port lock, to silence a sparse warning:

	drivers/tty/serial/pmac_zilog.c:255:36: sparse: sparse: context imbalance in 'pmz_receive_chars' - unexpected unlock
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NJohan Hovold <johan@kernel.org>
Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200723123327.5843-2-johan@kernel.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/pmac_zilog.c

@@ -213,6 +213,7 @@ static void pmz_interrupt_control(struct uart_pmac_port *uap, int enable)
 }
 
 static bool pmz_receive_chars(struct uart_pmac_port *uap)
+	__must_hold(&uap->port.lock)
 {
 	struct tty_port *port;
 	unsigned char ch, r1, drop, flag;