media: dvb_frontend: fix locking issues at dvb_frontend_get_event()

As warned by smatch:
	drivers/media/dvb-core/dvb_frontend.c:314 dvb_frontend_get_event() warn: inconsistent returns 'sem:&fepriv->sem'.
	  Locked on:   line 288
	               line 295
	               line 306
	               line 314
	  Unlocked on: line 303

The lock implementation for get event is wrong, as, if an
interrupt occurs, down_interruptible() will fail, and the
routine will call up() twice when userspace calls the ioctl
again.

The bad code is there since when Linux migrated to git, in
2005.

Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>