ANDROID: usb: gadget: f_accessory: fix CTS test stuck

f_accessory: fix CTS test stuck since CTS 9.0.
 - Refine acc_read() process.

The data length that user (test program) wants to read is different
from they really requested. This will cause the test flow stuck on the
2nd or the 3rd transfers in accessory test.
(By connecting 2 phones with CtsVerifier.apk and
CtsVerifierUSBCompanion.apk installed.)

Bug: 174729307

Change-Id: I5367c8075ed37534e8bed94b60cc79135ae5aebc
Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>