drivers/serial/sunsab.c: adjust the constant used to initialize the interrupt_mask0 fields

From: Julia Lawall <julia@diku.dk>

SAB82532_ISR0_TCD is declared in drivers/serial/subsab.h as relating to a
status register, while SAB82532_IMR0_TCD is declared in the same file as
relating to a mask register.  The latter seems more appropriate for the
interrupt_mask0 field, and follows the strategy for initializing this field
elsewhere in the same file.

Both SAB82532_ISR0_TCD and SAB82532_IMR0_TCD have the same value.

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>