serial: 8250: pruss: fix overflow in conversion build warning

Fix the build warning in arm64 build:

drivers/tty/serial/8250/8250_pruss.c: In function 'pruss8250_set_divisor':
drivers/tty/serial/8250/8250_pruss.c:131:41: warning: overflow in conversion from 'long unsigned int' to 'int' changes value from '18446744073709551614' to '-2' [-Woverflow]

Fixes: 57fe8746 ("serial: 8250: add pruss uart driver")

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>