usb/host/fotg210: convert macro to inline function

This patch convert the macro speed_char in an inline function. The goal
of this patch is to make the code easier to read.

Signed-off-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>