This patch adds the FEATURE_WRSR_WREN bit to the feature_bits for all supported Macronix SPI flash chips.

FWIW, the datasheets I used were for the MX25L1005/2005/4005/8005, MX25L1605D/3205D/6405D, MX25L3235D, MX25L512, MX25L2805D. All of them specify in the exact same wording that "Before sending WRSR instruction, the Write Enable (WREN) instruction must be decoded and executed to set the Write Enable Latch (WEL) bit in advance."