platform/chrome: cros_ec_chardev: Check ec_dev's availability in fops

It is possible that a userland program is waiting for further MKBP
events while the underlying ec_dev is already unregistered.

Check ec_dev's availability in the fops to avoid the userland program
from waiting for some events that should never happen.

Note: if the userland program uses poll() without a timeout, e.g.:
: fd = os.open('/dev/cros_ec', os.O_RDONLY)
: fcntl.ioctl(fd, 0xec02, 0xfff)
: p = select.poll()
: p.register(fd, select.POLLIN)
:
: while True:
:     es = p.poll()            # <--- without a timeout
:     if not es:
:         print("no events")
:         continue
:
:     for f, e in es:
:         if e & select.POLLIN:
:             data = os.read(f, 100)
:             print(data)

It'd still block.  The patch doesn't help with the case.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>