watchdog: mpc8xxx: Fix timer value

Timer value is a 16 bits calculated from the wanted timeout and the
system clock. On powerpc/8xx, a timeout of 2s gives a value which
is over U16_MAX so U16_MAX shall be used. But the calculation is
casted to u16 so at the end the result is 63770 instead of 128906.

So the timer gets loaded with 63770 instead of 65535. It is not
a big difference in that case, but lets make the code correct and
cast to u32 instead of u16.

Fixes: 26e8ebcd ("watchdog: mpc8xxx: Make it generic")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>