ppc: ppmc7xx: Fix possible out-of-bound access

ppc: ppmc7xx: Fix possible out-of-bound access

The flash_info_t->start[] field is limited in size by CONFIG_SYS_MAX_FLASH_SECT
macro, which is set to 19 for this board in the board config file. If we inspect
the board/ppmc7xx/flash.c closely, especially the flash_get_size() function, we
can notice the "switch ((long)flashtest)" at around line 80 having a few results
which will set flash_info_t->sector_count to value higher than 19, for example
"case AMD_ID_LV640U" will set it to 128. Notice that right underneath, iteration
over flash_info_t->start[] happens and the upper bound for the interation is
flash_info_t->sector_count. Now if the sector_count is 128 as it is for the
AMD_ID_LV640U case, but the CONFIG_SYS_MAX_FLASH_SECT limiting the start[] is
only 19, an access past the start[] array much happen. Moreover, during this
iteration, the field is written to, so memory corruption is inevitable.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Richard Danter <richard.danter@windriver.com>