usb_storage: fix ehci driver max transfer size

The commit 5dd95cf9 'usb_storage:
Fix EHCI "out of buffer pointers" with CD-ROM' introduce a bug in
usb_storage as it wrongly assumes that every transfer can use
4096 bytes per qt_buffer. This is wrong if the start address of
the data is not page aligned to 4096 bytes and leads to 'EHCI
timed out on TD' messages because of 'out of buffer pointers'
in ehci_td_buffer function.

The bug appears during load of a fragmented file and
read from or write to an unaligned memory address.

Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>