wifi: iwlegacy: Fixup allocation failure log

wifi: iwlegacy: Fixup allocation failure log

Fix 2 issues spotted by AI[0]:

1. Missing space after the full stop.

2. Wrong GFP flags are printed.

And also switch to %pGg for the GFP flags. This produces nice readable
output and decouples the format string from the size of gfp_t.

[0] https://sashiko.dev/#/patchset/20260319-gfp64-v1-0-2c73b8d42b7f%40google.com

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Link: https://patch.msgid.link/20260326-gfp64-v2-2-d916021cecdf@google.com
[add missing wifi: prefix]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>