peak_usb: fix clang build warning

Clang points out undefined behavior when building the pcan_usb_pro driver:

drivers/net/can/usb/peak_usb/pcan_usb_pro.c:136:15: error: passing an object that undergoes default argument promotion to 'va_start' has undefined behavior [-Werror,-Wvarargs]

Changing the function prototype to avoid argument promotion in the
varargs call avoids the warning, and should make this well-defined.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>