msm: ramdump: Don't fail read when !O_NONBLOCK

If the ramdump device node is opened in a blocking manner
(!O_NONBLOCK) then it makes sense to have that process sleep
waiting for the do_ramdump() function to be called in the kernel
instead of failing the read(2) call. This allows one to do
something like:

	cat /dev/ramdump_test

and then wait for the process to exit once do_ramdump() is called
and the read completes. The existing poll(2) interface is
unaffected by this change.

Change-Id: I9749e48f2561adae5a3f1bcb120a2aa4afcece82
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>