usb: gadget: fastboot: add max-download-size variable

Current Android Fastboot seems to use 'max-download-size' instead
of 'downloadsize' variable to indicate the maximum size of sparse
segments.

See function get_target_sparse_limit() in file fastboot/fastboot.c
in the AOSP:
	 https://android.googlesource.com/platform/system/core/+/master

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
usb: gadget: fastboot: explicitly set radix of maximum download size

The processing of the max-download-size variable requires a
radix specifier, or the fastboot host tool will interpret
it as an octal number.

See function get_target_sparse_limit() in file fastboot/fastboot.c
in the AOSP:
        https://android.googlesource.com/platform/system/core/+/master

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>