bnx2x: Improve code around bnx2x_tests_str_arr

This patch changes the definition of bnx2x_tests_str_arr from static char
pointer to static const char bi-directional array. Also the
bnx2x_get_strings function is simplified.

Reported-by: Joe Perches <joe@perches.com>
Reported-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Merav Sicron <meravs@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>