drv: ft8xx: extend single-letter function argument names

Replace single-letter function argument names in the FT8xx coprocessor
API with full names of the arguments for better code clarity.

The arguments in the ft8xx_reference_api.h are left as single-ltters to
keep the same function declarations as described in the FT8xx
programming guides.

Signed-off-by: Hubert Miś <hubert.mis@gmail.com>