iio: as3935: improve error reporting in as3935_event_work

gcc warns about a potentially uninitialized variable use
in as3935_event_work:

drivers/iio/proximity/as3935.c: In function ‘as3935_event_work’:
drivers/iio/proximity/as3935.c:231:6: error: ‘val’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

This case specifically happens when spi_w8r8() fails with a
negative return code. We check all other users of this function
except this one.

As the error is rather unlikely to happen after the device
has already been initialized, this just adds a dev_warn().
Another warning already exists in the same function, but is
missing a trailing '\n' character, so I'm fixing that too.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>