OMAP4+: HSI: hsi_char: fix write poll timeout issue

hsi_char driver may fail to return (POLLOUT | POLLWRNORM) after a write() has
occured. Instead it returns 0, causing the poll() to timeout.

This is due to a race condition between hsi_char_write() and if_hsi_notify().
Write callback can occur very fast in response to hsi_write() and may cause the
poll_event flag to be first set by if_hsi_notify() but then cleared by
hsi_char_write(), remaining to 0 forever and causing poll() timeout.

Fix is to clear the flag ahead of hsi_write to avoid this.

Change-Id: Iacd18f0448c1933a16b0a54db2204cd74fc259b2
Signed-off-by: Djamil Elaidi <d-elaidi@ti.com>
Reported-by: Christian Auby <christian.auby@stericsson.com>