i2c/i2c-pasemi.c: Fix unsigned return type

The function has an unsigned return type, but returns a negative constant
to indicate an error condition.  The result of calling the function is
always stored in a variable of type (signed) int, and thus unsigned can be
dropped from the return type.

A sematic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@exists@
identifier f;
constant C;
@@

 unsigned f(...)
 { <+...
*  return -C;
 ...+> }
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>