NFC: microread: Remove redundant assignment to variable err

In the case MICROREAD_CB_TYPE_READER_ALL clang reports a dead code
warning. The error code assigned to variable err is already passed
to async_cb(). The assignment is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Nigel Christian <nigel.l.christian@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>