tty: serial: switch mpc52xx_uart_int_{r,t}x_chars() to bool

tty: serial: switch mpc52xx_uart_int_{r,t}x_chars() to bool

mpc52xx_uart_int_rx_chars() returns unsigned int.
mpc52xx_uart_int_tx_chars() returns int.

The both results are binary ORed to the "keepgoing" variable. Unify all
three to bool as the only interesting value is whether we should keep
looping (true/false).

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220920052049.20507-7-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>