wifi: rtl8xxxu: Split up rtl8xxxu_identify_chip

Move the reusable parts into separate functions and create one
identify_chip function for each chip type.

This is preparation for supporting the RTL8710BU chip, which would
need too many ugly changes to this function. Another reason to do this
is to get rid of the long and scary if..else if..else block in the
middle of the function.

Everything should still work the same as before.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/b268b5cf-071c-6292-0d90-0573e4fb2228@gmail.com