mfd: omap-usb-host: Remove some unused functions

Removes some functions that are not used anywhere:
usbhs_readb() usbhs_writeb()

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Lee Jones <lee.jones@linaro.org>