staging: most: avoid null pointer dereference when iface is null

staging: most: avoid null pointer dereference when iface is null

In the case where the pointer iface is null then the reporting of this
error will dereference iface when printing an error message causing which
is not ideal.  Since the majority of callers to most_register_interface
report an error when -EINVAL is returned a simple fix is to just remove
the error message, I doubt it will be missed.

Addresses-Coverity: ("Dereference after null check")
Fixes: 57562a72414c ("Staging: most: add MOST driver's core module")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200624163957.11676-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>