MIPS: RB532: Fix set_latch_u5()

The data to be written is just a byte, so use writeb instead of writel.
Also, dev3.base contains the address, not the data so referencing here
is wrong.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>