usbif.h: replace PAGE_SIZE with USBIF_RING_SIZE

usbif.h: replace PAGE_SIZE with USBIF_RING_SIZE

Do not reference PAGE_SIZE directly: it could be undefined, or it could
have different values in the frontend or in the backend.

Define USBIF_RING_SIZE as 4096, assuming all users of usbif.h have 4K
page granularity. Replace PAGE_SIZE with USBIF_RING_SIZE.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>