Update spiflash._Bootstrap.transfer in response to changes in chipwhisperer
Update spiflash._Bootstrap.transfer in response to changes in chipwhisperer

This change updates _Bootstrap.transfer to work with the recent changes in
chipwhisperer (newaetech/chipwhisperer@68f12e1).

Signed-off-by: Alphan Ulusoy <23b245cc5a07aacf75a9db847b24c67dee1707bf@google.com>

lowRISC/ot-sca,lowRISC/ot-sca