arm64: Fix a use of update_lower_register

In get_kernel_reg_masked we use update_lower_register to get the lower
value of two registers for a given field. It will return the entire
register value with just the single field updated.

Because of this get_kernel_reg_masked needs to use the returned value
directly rather than ORing each field together. Fix this by updating
the mask and returning that from get_kernel_reg_masked.

Reviewed by: markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D40846