dahdi: Fix failure to read / write on kernel 3.16+

Kernel version 3.16+, since upstream commit (7f7f25e82d54870d "replace checking
for ->read/->aio_read presence with check in ->f_mode" )[1], does not like it
when dahdi changes the set of allowed file operations on a file descriptor
outside of the context of an open() system call.

DAHDI changes the available file operations when a channel is opened by first
opening /dev/dahdi/channel and then calling the DAHDI_SPECIFY ioctl to bind it
to a particular DAHDI channel. Until DAHDI_SPECIFY is called there weren't any
read()/write() callbacks implemented and therefore after the initial open, the
kernel was setting not setting FMODE_CAN_{WRITE,READ} on the file descriptor
indicating that those operations were not allowed.

Now define empty shell functions on the general dahdi_fops so the vfs layer will
not mark a file descriptor as unwritteable or unreadable on open.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7f7f25e82d54870df24d415a7007fbd327da027b

Internal-Issue-ID: DAHLIN-340
Reported-and-tested-by: Thomas B. Clark
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>