fix: wrong return type for _poll

See page 1055 of <https://uefi.org/sites/default/files/resources/UEFI_Spec_2_3_1.pdf>.

Author: wooster0

lib/std/os/uefi/protocols/managed_network_protocol.zig

@@ -15,7 +15,7 @@ pub const ManagedNetworkProtocol = extern struct {
     _transmit: std.meta.FnPtr(fn (*const ManagedNetworkProtocol, *const ManagedNetworkCompletionToken) callconv(.C) Status),
     _receive: std.meta.FnPtr(fn (*const ManagedNetworkProtocol, *const ManagedNetworkCompletionToken) callconv(.C) Status),
     _cancel: std.meta.FnPtr(fn (*const ManagedNetworkProtocol, ?*const ManagedNetworkCompletionToken) callconv(.C) Status),
-    _poll: std.meta.FnPtr(fn (*const ManagedNetworkProtocol) callconv(.C) usize),
+    _poll: std.meta.FnPtr(fn (*const ManagedNetworkProtocol) callconv(.C) Status),
 
     /// Returns the operational parameters for the current MNP child driver.
     /// May also support returning the underlying SNP driver mode data.