net: qed: use correct strncpy() size

passing the strlen() of the source string as the destination
length is pointless, and gcc-8 now warns about it:

drivers/net/ethernet/qlogic/qed/qed_debug.c: In function 'qed_grc_dump':
include/linux/string.h:253: error: 'strncpy' specified bound depends on the length of the source argument [-Werror=stringop-overflow=]

This changes qed_grc_dump_big_ram() to instead uses the length of
the destination buffer, and use strscpy() to guarantee nul-termination.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>