ar9331: reset_soc return the reset peripheral

1. passing a 0 to reset_soc will reset all peripheral
except the cpu itself.
2. return a value the peripheral reset.

Signed-off-by: Du Huanpeng <u74147@gmail.com>
ar9331 reset: use <linux/bitops.h>'s marco BIT

Signed-off-by: Du Huanpeng <u74147@gmail.com>