Bit 4: ClockEnSet the ClockEn bit high to enable an external clocking
(crystal or clock generator at XIN). Set the ClockEn bit to 0 to disable
clocking
Bit 1: CrystalEnSet the CrystalEn bit high to enable the crystal
oscillator. When using an external clock source at XIN, CrystalEn must
be set low.
The bit 4, MAX310X_CLKSRC_EXTCLK_BIT, should be set and was not.
This was required to make the MAX3107 with an external crystal on our
board able to send or receive data.