kernel: use struct group to wipe psb6970 volatile priv data

kernel: use struct group to wipe psb6970 volatile priv data

Instead of reference vlan and do strange subtraction, use the handy
struct_group() to create a virtual struct of the same size of the
members. This permits to have a more secure memset and fix compilation
warning in 6.1 where additional checks are done.

Fix compilation warning:
| inlined from 'psb6970_reset_switch' at drivers/net/phy/psb6970.c:275:2:
| ./include/linux/fortify-string.h:314:25: error: call to '__write_overflow_field'
| declared with attribute warning: detected write beyond size of field
| (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
|  314 |                         __write_overflow_field(p_size_field, size);
|      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|cc1: all warnings being treated as errors

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>