USBDevice: Silence GCC warning

The following line in USBHAL_KL25Z.cpp would generate a warning in GCC
because of a potential operator precendence issue:
    return((USB0->FRMNUML | (USB0->FRMNUMH << 8) & 0x07FF));

This would have been interpreted as:
    return((USB0->FRMNUML | ((USB0->FRMNUMH << 8) & 0x07FF)));
since & has higher precedence than |

I switched it to be:
    return((USB0->FRMNUML | (USB0->FRMNUMH << 8)) & 0x07FF);
Since it makes more sense to & with 0x7FF after having merged the lower
and upper bytes together rather than just the upper byte.  It should
have resulted in the same value either way.
Merge pull request #42 from adamgreen/usbdeviceGccWarnSilence

USBDevice: Silence GCC warning
Updates to GCC warning level flags

In gcc4mbed, I have been running with "-Wall -Wextra" and then
disabling a couple of noisy warnings that result.  In particular, I
disable the unused-parameter and missing-field-initializers warnings.
The first commonly goes off for implementation of virtual methods or
other overridable functions where not all parameters are required for
every override.  I don't find the second warning to be all that useful
anyway since missing structure field initializers will be set to 0
according to the C language specification.  The RTOS code uses this
language feature and I see no reason that it shouldn't :)
Merge pull request #43 from adamgreen/gccWarnFlagUpdate

Updates to GCC warning level flags