can: ti_hecc: fix -Woverflow compiler warning

can: ti_hecc: fix -Woverflow compiler warning

Fix below default (W=0) warning:

  drivers/net/can/ti_hecc.c: In function 'ti_hecc_start':
  drivers/net/can/ti_hecc.c:386:20: warning: conversion from 'long unsigned int' to 'u32' {aka 'unsigned int'} changes value from '18446744073709551599' to '4294967279' [-Woverflow]
    386 |         mbx_mask = ~BIT(HECC_RX_LAST_MBOX);
        |                    ^

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/20250715-can-compile-test-v2-1-f7fd566db86f@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>