at91samd: Remove 'first' and 'last' checks in 'samd_erase'

Remove 'first' and 'last' checks in 'samd_protect' since those
parameters are checked in 'handle_flash_erase_command'.

Change-Id: I30e5598a9ab656d81055f26cc63e291377605300
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-on: http://openocd.zylin.com/2024


Tested-by: jenkins
Reviewed-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>