flash/nor/stm32h7: (minor) use proper variable names and enhance logging

flash/nor/stm32h7: (minor) use proper variable names and enhance logging

-fsize_base should be fsize_addr as it is the address of FLASH_SIZE register
-flash_base should be flash_regs_base to avoid confusion with flash block start
-add LOG_ERROR to functions stm32x_[read|write]_flash_reg(...)

Change-Id: I86f035314bcd616fc0bdf445692d945a85c15481
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5362
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>