lib: sbi: Use mask to check the free bit during trigger allocation

The trigger allocation function uses bit shift instead of mask to check the
mapped status of the triggers. This causes index 0 to be return always. As a
result, the older triggers are overwritten.

Use the mask for MAPPED field in state word to check if the trigger is mapped.

Fixes: 97f234f1

 ("lib: sbi: Introduce the SBI debug triggers extension support")
Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>