flash/stm32h7x: remove IWDG1_SW separate variable

flash/stm32h7x: remove IWDG1_SW separate variable

Bit 4 in OPTSR is IWDG1_SW (the code originally called it IWDG1_HW, but
the reference manual refers to it as IWDG1_SW). This is broken out into
a separate variable, independent_watchdog_selection, in stm32x_options.
However, this is not necessary: bit 4 is included in the user_options
field, which includes all of bits 2 through 7, and
independent_watchdog_selection is not referenced anywhere else. Delete
the field and just rely on user_options to transport that bit, along
with all the other bits it contains, between stm32x_read_options and
stm32x_write_options.

Change-Id: I4da63df9272cf091267b956c412b95671ea1d3c9
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4744
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>