realtek: pcs: rtl930x: simplify DCVS/VTH/TAP calibration helpers

realtek: pcs: rtl930x: simplify DCVS/VTH/TAP calibration helpers

Reduce repetition in dcvs_manual, dcvs_get, vth_manual and tap_manual
by hoisting the per-channel enable bit write out of per-case branches
and encoding per-channel register/bit mappings as lookup tables where
the pattern is uniform across channels.

This also fixes two bugs in dcvs_get that were hidden in the switch:

- DCVS1 read never populated dcvs_sign_out; it wrote the sign bit read
  (0x14[4:4]) into dcvs_coef_bin and then immediately overwrote it with
  the coefficient read, leaving the sign always zero for that channel.

- DCVS3 assigned the manual bit read directly to a bool without the !!
  normalisation applied to all other cases.

Link: https://github.com/openwrt/openwrt/pull/23983
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>