firehose: Bump max payload size config parameter

The MaxPayloadSizeToTargetInBytes parameter of the configure command is
supposed to set the maximum size of the transfer unit. This size is
however limited by the use of the in-kernel qcserial driver and the use
of sector-sized writes. Yet increasing it, drastically bumps the
transfer speed, so leave it high.

NB. The max payload size supported by the device is part of the
configure response, if the device supports a value less than the
requested size - which we just ignore today.

Suggested-by: Kirill Kapranov <c_kkapra@qti.qualcomm.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>