libftdi: pass eeprom strings as const

libftdi: pass eeprom strings as const

These strings are not modified and it doesn't make sense that they ever
would be. Setting these as const ensures that they will not be modified
and allows const strings to be passed in without special manipulation.

Specifically a call like the following failed to compile:
ftdi_eeprom_set_strings(&ftdi, "foo", "bar", "abc123");

The error will look something like this:
error: ISO C++ forbids converting a string constant to ‘char*’
[-Werror=write-strings]