MIPS: RB532: Fix bit swapping in rb532_set_bit()

The algorithm works unconditionally. If bitval is one, the first line is
a no op and the second line sets the bit at offset position. Vice versa,
if bitval is zero, the first line clears the bit at offset position and
the second line is a no op.

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