staging: greybus: log: Fix line over 80 characters.

Fix coding style issue caught by checkpatch.pl related to the following
warning:
	- CHECK: WARNING: line over 80 characters
This is done by dropping a redundant cast and by replacing the format specifier
in dev_err(); to "%zu" instead of "%d", in order to silence the warnings of the
compiler.

Also, while at it, drop the redundant cast in the comparison as well to maintain
consistency.

Signed-off-by: Emmanuil Chatzipetru <chatzi.emanuel@gmail.com>
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>