serial: max310x: improve max310x_set_ref_clk() efficiency

serial: max310x: improve max310x_set_ref_clk() efficiency

The same basic code is duplicated for each of the four PLL multiplier
values, so simplify and streamline it. Doing so allows us to avoid
some unnecessary (fdiv * factor) multiplications for some fdiv values and
replace the second "if" statement with an "else if" and avoid a useless
comparison.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://patch.msgid.link/20260417-max310x-2-v1-5-b424e105ecac@dimonoff.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>