wifi: plfxlc: fix clang-specific fortify warning

wifi: plfxlc: fix clang-specific fortify warning

When compiling with clang 16.0.6 and CONFIG_FORTIFY_SOURCE=y, I've
noticed the following (somewhat confusing due to absence of an actual
source code location):

In file included from drivers/net/wireless/purelifi/plfxlc/mac.c:6:
In file included from ./include/linux/netdevice.h:24:
In file included from ./include/linux/timer.h:6:
In file included from ./include/linux/ktime.h:24:
In file included from ./include/linux/time.h:60:
In file included from ./include/linux/time32.h:13:
In file included from ./include/linux/timex.h:67:
In file included from ./arch/x86/include/asm/timex.h:5:
In file included from ./arch/x86/include/asm/processor.h:23:
In file included from ./arch/x86/include/asm/msr.h:11:
In file included from ./arch/x86/include/asm/cpumask.h:5:
In file included from ./include/linux/cpumask.h:12:
In file included from ./include/linux/bitmap.h:11:
In file included from ./include/linux/string.h:254:
./include/linux/fortify-string.h:592:4: warning: call to '__read_overflow2_field'
declared with 'warning' attribute: detected read beyond size of field (2nd
parameter); maybe use struct_group()? [-Wattribute-warning]
                        __read_overflow2_field(q_size_field, size);

The compiler actually complains on 'plfxlc_get_et_strings()' where
fortification logic inteprets call to 'memcpy()' as an attempt to copy
the whole 'et_strings' array from its first member and so issues an
overread warning. This warning may be silenced by passing an address
of the whole array and not the first member to 'memcpy()'.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230829094541.234751-1-dmantipov@yandex.ru