lib: sbi: Put event after use in sbi_sse_exit() loop

Currently, the sbi_sse_exit() gets event in a loop but does not put
it back after use. This results in global events remaining locked
causing hangs on sub-sequent calls to sse_event_get() for global
events.

Fixes: c8cdf01d

 ("lib: sbi: Add support for Supervisor Software Events extension")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Himanshu Chauhan <hchauhan@ventanamicro.com>