Remove ARM pgm_read_word workaround in rgblight

[zvecr]

Description

Reproduced in deb10 + arm gcc 6.3.1. Error was

Compiling: quantum/rgblight/rgblight.c                                                             quantum/rgblight/rgblight.c: In function 'rgblight_sethsv_eeprom_helper':
quantum/rgblight/rgblight.c:563:17: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
                 uint8_t range = pgm_read_word(&RGBLED_GRADIENT_RANGES[delta / 2]);
                 ^~~~~~~
cc1: all warnings being treated as errors

This was due to the wrong progmem api being used, trying to read a u8 with th u16 version.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).