flash/nor/tcl.c: Do not double probe banks

Previous to this version the code of handle_flash_probe_command would
probe a bank twice: first time by auto-probe through a call to
flash_command_get_bank and second time by calling the probe function
directly. This change adds a flash_command_get_bank_maybe_probe wich
is a more generic version of the flash_command_get_bank, that would
allow commands to decide whether auto-probing should be performed or
not.

Change-Id: If150ca9c169ffe05e8c7eba36338d333360811e3
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-on: http://openocd.zylin.com/2093
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>