First of all, thank you for answering Chris.

I've just downloaded the last sdcc version and I've checked the assembler code and it seems to be right too:
_EPINDEX = 0x00f1
...
; as_usb.c:56: EPINDEX=0xF8;
...
mov _EPINDEX,#0xF8
...
; as_usb.c:57: aux=EPINDEX;
...
mov r2,_EPINDEX
...
; as_usb.c:58: aux++;
...
inc r2

But the value in aux is 0xF2! instead of 0xF9. It doesn't seem to be a problem of SDCC.
Anyway, I've sent a mail to Winbond asking them about it.
As soon as I solve this problem I will post the solution.

Thank you,

Xavier