mfd: omap-usb-tll: Fix cppcheck sizeof warning

Static analysis from cppcheck issued the following warning:

[drivers/mfd/omap-usb-tll.c:255]: (warning) Found calculation
  inside sizeof().

The current size calculation is not obvious and is easy to
miscomprehend, so re-work the size of the allocation based
on the size of the struct pointer and quantity to allocate.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>