drivers: winc1500: Fix signed/unsigned comparison

The value from socket() was directly assigned to offload_context, which
was treated as a unsigned integer when compared. This prevented the
following if statement from catching any errors, leading to random RAM
access. This is fixed by using an intermediate with the same type as the
return value, which is then assigned to offload_context after error
checking.

Signed-off-by: Abram Early <abram.early@gmail.com>