wifi: mt7601u: fix clang -Wformat warning

When building with Clang we encounter this warning:
| drivers/net/wireless/mediatek/mt7601u/debugfs.c:92:6: error: format
| specifies type 'unsigned char' but the argument has type 'int'
| [-Werror,-Wformat] dev->ee->reg.start + dev->ee->reg.num - 1);

The format specifier used is `%hhu` which describes a u8. Both
`dev->ee->reg.start` and `.num` are u8 as well. However, the expression
as a whole is promoted to an int as you cannot get smaller-than-int from
addition. Therefore, to fix the warning, use the promoted-to-type's
format specifier -- in this case `%d`.

example:
```
uint8_t a = 4, b = 7;
int size = sizeof(a + b - 1);
printf("%d\n", size);
// output: 4
```

See more:
(https://wiki.sei.cmu.edu/confluence/display/c/INT02-C.+Understand+integer+conversion+rules)
"Integer types smaller than int are promoted when an operation is
performed on them. If all values of the original type can be represented
as an int, the value of the smaller type is converted to an int;
otherwise, it is converted to an unsigned int."

Signed-off-by: Justin Stitt <justinstitt@google.com>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220711212932.1501592-1-justinstitt@google.com