net: macb: replace min() with umin() calls

net: macb: replace min() with umin() calls

Whenever min(a, b) is used with a and b unsigned variables or literals,
`make W=2` complains. Change four min() calls into umin().

stderr extract (GCC 11.2.0, MIPS Codescape):

./include/linux/minmax.h:68:57: warning: comparison is always true due
                          to limited range of data type [-Wtype-limits]
   68 | #define __is_nonneg(ux) statically_true((long long)(ux) >= 0)
      |                                                         ^~
drivers/net/ethernet/cadence/macb_main.c:2299:26: note: in expansion of
                                                            macro ‘min’
 2299 |              hdrlen = min(skb_headlen(skb), bp->max_tx_length);
      |                       ^~~

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://patch.msgid.link/20251014-macb-cleanup-v1-11-31cd266e22cd@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>