staging/btmtk_usb: Mark local functions as static

sparse complains about some local functions not being static:
drivers/staging/btmtk_usb/btmtk_usb.c:50:6:
warning: symbol 'hex_dump' was not declared. Should it be static?
drivers/staging/btmtk_usb/btmtk_usb.c:227:5:
warning: symbol 'checksume16' was not declared. Should it be static?

-> add the static keyword

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>