mfd: omap-usb-tll: Remove unused usbtll_readb() function

Clang with W=1 reports:

  drivers/mfd/omap-usb-tll.c:128:18: error: unused function
    'usbtll_readb' [-Werror,-Wunused-function]
  static inline u8 usbtll_readb(void __iomem *base, u32 reg)
                 ^
This function is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230322125803.2570968-1-trix@redhat.com