cfi: fix write_bank segfault with spansion flash on armv7m

cfi: fix write_bank segfault with spansion flash on armv7m

cfi_spansion_write_block() passes an arm_algorithm struct to
target_run_algorithm() which in turn calls armv7m_start_algorithm()
which expect an armv7m_algorithm struct.
As armv7m_algorithm is bigger than arm_algorithm, when
armv7m_start_algorithm() writes in the struct, it overrun the buffer,
writting junk on the stack, which latter on generates a segfault.

This patch ensure we use a properly sized armv7m_algorithm struct
when the target is an armv7m.

Change-Id: I4ab67c15ae4bb72454414a81b92a4231dcdb2239
Signed-off-by: Aurelien Jacobs <aurel@gnuage.org>
Reviewed-on: http://openocd.zylin.com/623
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>