usb: musb: Fix a few off-by-one lengths

!strncmp(buf, "force host", 9) is true if and only if buf starts with
"force hos". This was obviously not what was intended. The same error
exists for "force full-speed", "force high-speed" and "test
packet". Using strstarts avoids the error-prone hardcoding of the
prefix length.

For consistency, also change the other occurences of the !strncmp
idiom.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Felipe Balbi <balbi@ti.com>